I have a very similar model, RM's KL-203P (added preamp). Currently getting good SWR under all modes (preamp, no preamp, AM, SSB, powered off), but I'm not getting any output gain at all when engaged (AM or SSB).
What does work is signal pass through, 3W in, 3W out in all modes. The preamp is operational (though arguably equivalent to a noise booster). Also, the SSB delay circuit behaves a expected for- both R/T, just no forward gain.
I don't currently have a dummy load for bench testing, but I do have an ESD safe bench and soldering equipment (my current job is in digital, rather than RF), so I did a "blind" swap of the 4 MOSFETs with IRF520N, based on other accounts of failing MOSFETs having similar results. Unfortunately, still no measurable forward gain (on the good side, SWR, etc. didn't worsen, and I got the MOSFETs regreased).
Are there any pertinent standby measurements (components, etc.) I could test on this unit prior to getting an adequate dummy load for testing transmission?