x-90 P0335 code

[fixkick]

or that CKP analog port is weak, some ECU may be more sensitive on the CKP port more noise immune or both.
CKP is the weakest sensor on the for sure at idle slow RPM.

why not just run the tracker ECU, it may be happy in the X90... if it just plugs in , im sure no harm can come. (real harm)
see of you get other codes, like EGR with wrong ECU. that can happen.
your car is 5speed manual stick shift so no 3/4speed AT horrors.. TCC and autoshifted, 4speed,.
your set up is simple with 5speed.

I do feel you pain with 335, but never see this car not possible to fix with a new CKP or no pan gasket. there.