I take care of a friend's bike. He brings it over once or twice a year. It is a lower end Univega with plastic 5 speed Shimano "SIS" stem shifters. He rides it every day, year round, to get around. He's not a "bike person" at all, never maintains his bike, the Univega is just transportation. But it is a good city ride, with fenders, LED lights, sidewall generator with lights, rear rack, wide cushy tires, Koolstop brake pads, too ugly to steal.

When he brings it to me, I adjust the brakes and shifting, lube everything, true the wheels, fix any issues he has. Last year I replaced his worn out front rim last year with a take-off from one of my projects. I know it won't be cleaned or lubed for the next 6 to 12 months - sad - but it gets him around.

The bike rides nicely. But his drivetrain is close to worn out. The freewheel teeth are visibly hogged out, the inner chainring teeth are pointy. I didn't measure the chain, but I'd guess it is elongated. He's not having any chain skip, but I decided not to replace his chain because I think that will result in skipping where he now has none. Also, I think the shifter indexing is getting worn. I can no longer adjust it to shift cleanly; adjustment for the upshifts isn't exactly the same as adjustments for the downshifts.

I told him to bring it back before the winter starts and I'll do a more thorough service. That will give me time to rummage through co-op bins for cheap parts. Yeah, he is a really good friend.

My question is, should I find replacement parts just like what he has now?. Which might be hard, finding a 5 speed SIS compatible freewheel. (Edit: err, looks like 5 speed Sunrace freewheels cost $13 new. But what about replacement shifters?)

Or should I upgrade him to a more modern drivetrain?. 7 or 8 speed, bar end shifters, even cheap brifters? The budget will be about $100. Assume everything (but consumables) will be bought used and cheap.