You found a 90 TRX350 engine or a 300 engine?
the 86-87 Fourtrax use a different stator, CDI, starter switch assy, oil sensor harness and decompression assist. So if you try to bolt up a 90 TRX350D engine, you will have to account for those differences. Maybe your old engine can be a donor for the stator, oil sensor—not sure about the decomp lever.
I don’t think you can bolt up a 90 TRX300 engine