have reverse but not fwd gear

[cseany66vw]

I have a 4cyl mercruiser 140 gen one outdrive...the upper unit burned up, gears stripped. Lack of gear oil as I did not find any remaining after I took it apart. The engine also was over heating at the time i lost the upper unit. I removed lower unit in nuetral. replaced upper unit, intalled lower unit. I can get reverse but not forward gear. If I put the throttle all the way forward it does get some forward gear. noisy. Do I need to dissassemble and put lower unit and gear select into forward and then put back together? And any ideas on where all my gear oil went and the overheat? Thanks!

Sean