capnjonny 

In my garage shop I have at least one of almost every derailleur Suntour ever built. I have cleaned and reused lots of them on bikes I have restored for the Bike Exchange. In all that time I have seen maybe 5 that were damaged . Unless you want to go with index shifting I see no reason to switch to Shimano. What I would highly recommend is to change that freewheel to a new Shimano Hyperglide. You will get greatly improved shifting. With the old Suntour freewheel you may experience over shifting where the chain goes from say 3rd position to 5th instead of 4th . The Hyperglide freewheel will probably eliminate this entirely. the ramps on the cogs will catch the chain with just a nudge of the shift lever and pop it onto the next cog. I second the recommendation for a new KMC 8 spd chain if you do switch Freewheels. doing so will enhance the shifting even more. I noticed you have the spring loaded Suntour ratchet stem shifter. That is the best stem shifter out there. the spring loaded ratchet mech makes shifting effortless. The only other change I would make would be to change out those bars for a set of 2" riser bars and a set of Suntour ratchet thumb shifters (Sunrace makes a very inexpensive set of friction thumbies that also work great if you can't find the Suntour set) . This lets you keep your hands on the bars at all times even when you are shifting and also lets you sit more upright which at my advanced age is more comfortable.