i/o motor shifting problems

[socalboater]

i own a 1970 sea ray the valve cover says mercruiser 120hp i was wondering if that is a stock motor set up because the motor and out drive are very clean and rust free but my problem is that the foward gear is perfectly ok and nuetral is ok but when i go to put it in reverse it just wont go in gear finaly when it goes into gear it very noisy sounds terible and ive ben running it like that will that cause any problems to my foward gear any suggestins will be greatly apriciated thanks