m42 head with m44 block
there are a couple approaches to this-
user "1991 E30 M42" (i think that's the handle) swapped a complete M44 rotating assembly into an M42 block, making everything plug-and-play.
advantage: freshly built motor. disadvantage: more expensive.
another user whose handle i forget put an M44 block under his existing head. search on those keywords and you should be able to find it. the M44 block only has a spot for one coolant temp sensor whereas the M42 has two, so he had to relocate one of them to the passenger-side coolant hose. other than that, he seemed to have an easy time of it- if you're competent enough to r&r a cyl head, you should be fine.
advantage: less expensive. disadvantage: used motor.
using the entire M44 engine would require the M44 wiring harness and ECU as well, and it's OBD2- you don't want to go this route; much more complication and hassle for no appreciable gain.