The best thing to do is put a smal turbo on it.

D series all motor, waste of time. An intake manifold, MILD Cam (most of the cam profiles suck), and a ok header might get you a 10 whp gain but highly unlikely.

The best thing you can do for a SOHC to make it feel good is a LIGHT flywheel and a closer geared transmission.

12.5:1 or 13.5:1, have a good time paying for race gas.

Compression isnt where the power is.