force 125 run OK but no reverse

My force 125 hp runs but wont go into reverse, it will go into forward, nuetral and reverse easily. shifter and cables working fine. gear case oil was sampled and found no emulsion and oil was fairly clear, slightly darkened (perhaps not changed for years), some small traces of metal (size of the dot this "i"). Is there a quick way to determine whether this is shifter shaft adjustment issue or a problem in the gear case on. The power head was replaced with a used power 1 year ago psi's yesterday 125, 125, 115, 120, 1 > 4. engine runs OK. studied the repair manual and looks like the power head matches the 1989 and later sketch. I noticed that the cowl has u s marine logo on the front side suggesting that this unit maybe early-mid 1980's. getting back to the issue of reverse, i wonder how to check to see if when this newer version power head was installed with proper shift rod adjustment or if .there was an improper conversion during installation which would not allow full engagement into reverse. Also i noticed that the shift shaft in reverse is making solid contact against the bottom of the crankcase casting. Could this mean excess wear to the clutch and, or yoke; or do i need a different length shift shaft/part no. to rectifiy the conversion to the newer power head. Comments and suggestions please