Not many people have done a turbo m42, a few have been documented on this forum if you search deep enough. Typically for a small displacement 4 cylinder the turbo I would look for that suits you power goals would be a T3 style in the 50mm range with a .63 A/R on the exhaust housing. There is an exhaust manifold and downpipe made by OBX (
https://www.ebay.com/itm/OBX-Turbo-M...-/162269228973) however I've heard bad things about it's quality. Injector size depends on if you upgrade your fuel pump, it should be comparable to turbo M50/52/S50/52 injectors. Depending on how low you go with compression will dictate how much boost you can run. If you're looking to run high boost (15+ psi) you should go with a conservative CR of 8.5:1 or 8.8:1. You also have o consider the higher boost you run the greater chance of pushing oil past the piston rings, so if high boost you'll have to do pistons as well. LOTS to consider. VAC Motorsports makes and supplies a lot of M42/44 engine internals.