Two quick thoughts.
While the $600 or so that a transmission overhaul seems to cost from one of the gurus is a chunk of change, particularly given the value of an R65, I'd have to be certain about either the price advantage or the provenance of a "good used" unit before going down that route.
When I was working, I’d sometimes experiment on how long the battery would hold up on my R65 having to deal with my 3-mile to/from work sprint, including home for lunch most days. I could sometimes squeeze two weeks, but never more. I wired a lead with an SAE socket to the battery, cable tied the socket to the frame just below the left side panel and got into the habit of simply hooking it up at work and at home. No problems.
Mark