Upgrading 94 Norco Bigfoot frankenbike with old SunTour components
I bought a (94-ish) Norco Bigfoot to use as a winter bike, and also to sort of experiment on. I thought it would be fun to buy an old bike with a solid rustless frame and kind of play around with some low level upgrades here and there, just to teach myself some basic bicycle mechanics.
When I got home and had some time on the internet to try to figure out exactly what year of bike I had, and what the component specs were, I realized that a lot of the stock components had been swapped out with vintage SunTour components from the 70's.
Now, I actually think this is kind of cool - the components look to be practically brand new and they work beautifully. Friction shifters (SunTour Mighty Shifter, I think, which was their cheapest), GT rear deraileur, and a 5 speed freewheel. Also SunTour front der, but i can't remember which model off the top of my head.
I'm thinking I'll keep the friction shifters and derailleurs, but I'm wondering about that 5 speed freewheel. This type of bike never came with anything smaller than a 7 speed (cassette, I think) which makes me think that the 5 speed must be too small for the frame? I wouldn't mind upgrading to a 6 or 7 speed, but from what I've read (on forums like this) the size difference from a 5 to a 7 is prohibitive. But as far as I can tell the original bike should have come with a stock 7 speed. Am I out to lunch or should it be easy to re-upgrade?
I guess my question is - should I leave well enough alone, or should I drop in a new freewheel. If so, anything I should know when looking at a replacement? (Such as which models work well with the SunTour GT rear der, which speed I should get, what should I do or not do as I'm switching it out etc). I'd like to be able to put a new one in without having to monkey around with the frame too much.
As an aside, I could swear that the guy I bought this from had no idea that there were replacement parts. But who other than someone with a particular fondness for vintage bikes would downgrade from indexed shifting, 7 speed cassette to friction shifting 5 speed.