Just pondering through other reasons for your reverse issues.
Does your lever controlling forward and reverse have full travel?
I think if it were me I would do a study of linkages and anything that pivots.
Start at the hydro lever and go to the hydro and lube it all. It is amazing
what some maintenance will change. And if you do that and still have troubles
you can start tearing into things. And besides, then things will be all lubed
up and ready to go.
Ken