well, the good news is that there are enough disloyal ingrates who swap their M42 stuff out for other engines
so if you want a new engine, they show up rather frequently, sometimes for nothing or next to nothing!
There's a lot of guys here that have cylinder heads and/or cylinder head parts... they will chime in!
As a rough benchmark, bavarian engine exchange (not really recommended) sells M42 longblocks on ebay for around $900...
...and welcome!