There is no specific large-bore (1275cc-based) head casting number that will fit the 998 engine any easier than any other. They all have the same problems - generally requiring exhaust valve relief cut-outs machined into the block and re-alignment of the front water gallery transfer ports. However, I would avoid using heads with valves bigger than 35.7mm on the intake since these are too big for most 998 engines unless they are absolutely full-race spec where top end power at high rpm is all that is required.
First of all it is worth checking that you actually need to make the relief cut-outs since some heads have a big enough head face to exhaust valve face clearance to allow fitment without the cut-outs when the standard cam and rocker gear is retained.
If possible, measure the actual valve lift you are getting with the current fitted set-up. Carefully set the valve lash clearance (clearance between valve tip and rocker) to 0.012-in./0.30mm. With an exhaust valve in the fully closed position, position a dial gauge (DTI) on the valve cap, zero the gauge and then rotate the engine until maximum valve lift is achieved - counting the amount of lift indicated by the gauge. This is likely to be around 0.270-in./6.86mm. The maximum you are likely to see is 0.280-in./7.11mm if you have a particularly good set-up since the standard 998 camshaft will only have 0.235-in./5.97mm lift at the lobe, the maximum rocker ratio is likely to be 1.25 - the valve lash setting being taken from the result. So 0.235-in. x 1.25 = 0.294-in., less the 0.012-in. lash setting gives 0.282-in. BUT it is rare that the standard sintered steel rockers give anything like this lift ratio. From personal experience the best to be expected is 1.22 - so the actual nett full lift will be less.
If you can't check the valve lift because the head has already been removed, you will need to check the lift at the cam using a DTI positioned on a push rod placed in one of the exhaust valve followers riding on the camshaft whatever reading you get, multiply this by the rocker ratio and minus the valve lash setting detailed above.
With the 1275 head upside down, accurately measure the depth of the exhaust valve face in comparison to the head face. This measurement needs to be at the very least the same as the exhaust valve lift registered as above. Preferably greater. It is possible to increase this distance by cutting the exhaust valve seats a little further into the head and/or removing material from the face of the exhaust valve. I would only look to gain around 0.030-in./0.76mm by doing this though. Alternatively, going to a race-spec exhaust valve may nett some more clearance since the valve seat edge to valve head face on race valves tends to be less than standard OE type valves. If doing this when using the standard cast iron guides, it is essentially you use either a chromed-stem type competition valve or one of the later nitrocarborised/Tuftrided type valves (charcoal grey/black in appearance.
All of the above is totally affected by camshaft type and rocker gear used. 1.5 ratio rockers simply don't work in general on road-going 998s in my experience so should be avoided. The probable maximum cam lift you may be able to entertain using the above criteria would be 0.250-in./6.35mm as used on the more 'performance' standard profiles such as the MG Metro, standard 1275 Metro and older 998 Cooper and S cams. In which case you'd be looking for around 0.293-in./7.44mm clearance between exhaust valve face and head face to avert using cut-outs.
The head gasket is your safety margin - so don't add this into the equation at any point. Crushed as fitted it gives around 0.028-in./0.71mm to 0.032-in./0.81mm thickness - just enough for that safety margin. Don't ever over-rev the engine or miss a gear-shift though!
And talking of head gaskets - you will need to use the 1275 item to get a reasonable alignment for sealing both the combustion chambers and the front water gallery transfer ports in the head. The 998 head gasket over-hangs the 1275 head combustion chambers on the exhaust sides and doesn't give as good a line-up of those water gallery ports.
It is prudent to plug the front water gallery transfer port holes and re-drill them using a 1275 head gasket as a template. This is pretty easy even with the engine in the car by tapping the holes then fitting suitably sized grub screws which are then drilled to allow water through in line with the 1275 gasket. Crude but effective and means the block face doesn't need re-surfacing afterwards. If this realignment isn't done, the head gasket is likely to leak water out and down the front of the engine.
If cut-outs are necessary in the block, the easiest way to establish their relative position is to use the 998 head gasket. Place on the block using head studs for alignment. Scribe around the outer exhaust edges of the gasket onto the block surface. Use a fly-cutter of suitable size to machine the cutouts in. It is possible to use a die grinder to make these cutouts, but only in desperation! The depth needs to be sufficient to clear however far passed the head face the exhaust valve will be when the valve is at full lift. To establish this simply subtract the exhaust valve face to head face measurement from the full lift measurement. To be safe I would add 0.040-in./1mm to this depth to ensure no valve to block contact despite using the gasket as a safety margin. And avoid going so deep as to intrude on the top ring land.