To address some of your questions on repairing your 1650, the tail light lenses are somewhat hard to come by, but you can sometimes find them on ebay. I did purchase a NOS pair from Lew (darkminion17) on this forum that he had put up on ebay. He might have some more. You could PM him or he might chime in here shortly.
The lenses are a bit easier to find than the entire fixture, but they often have cracks or chips in them. As for the bulb and socket, I'd go to NAPA or similar and find a fixture you can rob the socket from. They would also have the bulbs. Most of the new fixtures today are going to LED, so you might have to look around a bit.
The iso mounts definitely are a trouble spot, so get those addressed and I believe that the IH/CC replacement mounts are the best, but they are pricey. There are aftermarket mounts that cost less. I usually go to ccspecialties.org for parts. There is an article in the tech library on how to do the cradle mod. Might as well do that if you can while you have the engine out to do the iso mounts. look here:
https://www.onlycubcadets.net/forum/...ead.php?t=4630
There are a lot of other articles in the tech library. Might want to change fluid and filter if it hasn't been recently. Open the tunnel cover (between the seat and the dash) and check the fan. It's plastic and often damaged. Clean any debris off the hydro and rear end. Look at the trunnion and see if it needs attention. fixing wear on the trunnion and replacing the springs does a lot to control speed up and down slopes. Look for the hydro lurch article in the tech library.
Put some new rubber on that puppy and she'll look like new.
Here's my 1650.