Most of the ones I can think of are already covered, but here's another:
Subaru EJ18...an EJ22 swaps in with literally zero other changes (even the ECM works) and you can get perfectly fine used EJ18s all day long for $100-200. People pull fine working EJ18/22 out of older Subies and put in EJ20/EJ25 turbo motors in because the Subies are like legos.
Bryce

