Simplex with new gears and an IWIS chain - stronger and less rotating mass.  Swiftune do an awesome kit if you're feeling flush.

Fuel pump breathers are generally rubbish in my experience.  Gertie has had both fitted and she breathes far better with the timing cover one instead of the fuel pump one, which if baffled to prevent wet oil being sucked/thrown out negates the point of having it.

You mention building a 1293 and using parts from a 998. The 998 s I have been working with have a breather on the tappet cover and not the timing cover like you'd expect on a closed back 1275 block. Also depending on what clutch housing ( most 998 s have the breather opening in the clutch housing closed ) the only breathing a closed back 1275 has is timing cover , valve cover and clutch housing breather. You can rig something from the fuel pump opening but mostly racers do that. For most street power units I buy new simplex gears and a Iwis chain as mentioned. I have also begun replacing all timing covers with late timing covers with the smashed breather can.  You will find that if you convert to duplex timing set up, there is a shield around the breather opening that has to be removed and a small dimple that has to be bumped out. Also the front engine plate is different depending on weather tensioner is fitted or not. 

You may notice that some suppliers are fitting simplex timing kits to their race engines. They are not as pretty as the adjustable duplex kits but once adjusted and sealed you can't see it anyway. Steve (CTR) 

Existing cover will work, just need to counter sink for 2 Allen keyed machine screws vs. raised bolt heads for clearance on the front cover.

If duplex gears/chains you can still use tensioner but if good chains like IWIS or REPCO not needed.

• I’m putting together a fast road 1293 (using parts from my 998) and I’m contemplating replacing the single timing gear/chain.  It’s not that expensive, but is it worth it?  Do I need a new cover? Or will the existing cover work?
• Jamie