an M44 (1900cc IIRC) with a different intake setup, maybe throttles (sidedraughts would be fine on a track), a 285-ish set of cams, a little off the head to slightly raise compression and a decent exhaust would push out about 130kw (flywheel) which is right up there with the (stock) sixes... This is general though and you would have to get the specs of the cams, find out how much you can realistically raise the compression, and check out a few things... but still very doable.
you wont get the torque of the sixes but it wont be a huge disadvantage on a track as you would be able to keep the motor in its (probably narrow) powerband. and realistically you would be removing about 40kg from the front axels... it would be a close run thing, M44 modded vs. stock 325/328... but i know i would have more fun in a four pot zinger...
i managed to get hold of one quite easily.. but that was a while ago now.. maybe someone else can help here... also depends on budget - throttles, cams, management and exhaust can easily cost 10k...
more and more i feel like you cant have anything close to what you want without spending 3 times what you think it should cost.... lol