I like your decision to try and repair and use what you have. Over the last year I have mostly ridden on a 1973-vintage bike with a Shimano Eagle and it has worked perfectly after I made sure it was really clean. I have a couple old bikes I ride with 5-speed rear freewheels and two-speeds up front, and the standard rear freewheel on most of these bikes, which I think is a 14-28 makes it very easy to go up steep hills. On one of the bikes I put a 15-24 on the rear and it made it noticeably harder to climb steep hills with. Where I live it is pretty easy to find old ten-speed parts, and I have been making the effort to collect some spare parts, most of them end up being scrapped, the last ten-speed I bought, for $5, was right out of the back of a scrapper's truck while he was driving around on trash-day picking up metal to cash in. I would not be surprised if some of the derailleur wheels on the common old equipment would be interchangeable, but I have not had to try it yet. I suspect that the plastic wheels on many derailleurs are self-lubricating and might run best dry, but I put them together with the tiniest bit of light grease I can, and if you oil your chain regularly they get lots of oil on them eventually. A lot of old bikes get slow because the derailleur wheels get old sticky dirt and grease in the wheels, so I think one of the most important things to do is to keep that derailleur clean, and of course the bottom-bracket too. These old manufacturers put lots of research into what consumers wanted and needed, so I would certainly try the bike as it left the factory before I tried changes. I don't think anyone who rode every day or two would need to change the factory gearing unless they lived in a mountainous area.