most of the parts that wear can be gotten in one way or another either from honda, other vendors or can be adapted, some of those parts are cheap (like seals, o-rings, etc…) but it’s a lot of effort to replace them, so you might as well overhaul or replace other parts while you have that part of the bike taken apart.

the parts that don’t wear shouldn’t need replacing, unless you wreck or damage it. those parts can be found used online, motorcycle dissmantlers, etc. they are likely to be pricey if you can still get them from honda.

as far as econonmics. bikes are relatively cheap to operate, i.e. cost/mile. so even if you have to overhaul a 25year old bike, after you’re done there is no reason it can’t last another 25 years. the xl600r was like $2300 new wen it came out. you can buy one for $1500 or less now, and even if you have to spend another $1000 to totally overhaul it it’s still cheaper then a new bike, you can get a lot of miles out of it.

of course, there are some parts that won’t last long on a bike. rear tires last about 3000-5000miles ($50-75/ea.), a good o-ring chain will last 8000-10,000 miles on the road ($100+). the trick there is to replace it before it stretches and ruins the sprockets. brakes last a while, if you don’t ride in the mountains. batteries, for some reason don’t last very long.

some of the older bikes are actually compare very well to newer bikes in terms of performance. the xl600r is one of them, as long as you don’t mind kick start. at least you can ride it without the battery, if need be and it gets really good mileage for that size bike.