There really isnt a whole lot of aftermarket engine parts for the m42 in the us. i know theres more in the euro market but the biggest here would be metric mechanic or vac motorsports. both are very expensive. everything in my m42 turbo car came from vac. runs like a champ. you could easily see 200whp with a stock m42 and boost. our bottom end is pretty bulletproof and top is pretty solid as well. 10psi on a stock m42 should see 200whp i would assume. on 9psi with 8.0:1 cr my cars at 201whp and 198wtq. with stock compression you should beat that.
Another route is boring for m/s5x pistons and increase stroke with an m44 crank. mix and match parts from other engines to create a little monster. But if you want to spin it at 9000rpms... Id keep the stock crank, maybe 85mm pistons, lower compression if you decide to go turbo, lighten up the valve train and talk to catcams about making a cam for you thats good up to 9k. i know mm has a cam for 8k.
and youll want to do something about the trans too. g240 will take a little abuse but i imagine youll kill it pretty quickly. and dont waste your time going to a g260. ive had nothing but problems with my swap. use a zf trans and find a recipe that someone else has used and that is proven to work. know exactly what tob, pinion bearing, etc. itll save you a lot of headache. trust me there.