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 a brief word on head gaskets. If it is not a genuine Rover or Payen gasket, don’t use it.

On 2" inch main 1275 engines the use of main studs gives a solid improvement over the stock bolts. Factory bolts, at the very best, only have 65% of the clamping force of an ARP stud, and consider that the combination of the firing load and inertia at RPM cause loading's that amount to thousands of pounds. On the 1275 Mini there is something to look out for that doesn't arise on an inline engine. Due to the proximity of the gear box webs at both ends, there is not enough clearance for the nut.