Reverse gear problem

[Monkey's fist]

I have a 4HP 2 stroke that has suddenly stopped going into reverse. Prop spins when shift lever is in neutral and engages when in forward - but spins when in reverse. I removed the prop drive and can see the lower unit linkage working ok in both directions - with the pin out of the lower unit shift lever works fine. Once reassembled wont go into reverse. Wondering if there is wear in the lower unit linkage faces? How easy to replace? Note: Lower unit was full of water but all gears and teeth seem fine. Any advice appreciated.

[boats.net]

Sounds like have problem inside gearcase, need to check shifter parts and gear in gearcase.