Disclaimer: I am relating my own RD experience, not attempting to discredit anyone.
I bought my 77 RD 400 in 2001 with around 2000 miles on it. It now has just turned 6,200. I know for a fact that my bike was not ridden 100/miles a year from when it was new, it sat for years somewhere with no use before I bought it. My RD250 was run around my great uncle's backyard for a long time and had only 8k miles when I sold it, and the crank seal was fine.
While I have not done a leakdown test on my 400, it runs pretty well, plugs are always perfect and no problems/symptoms of a lean condition. A lot of posts say 5k miles is the magic number for crank seal failure. I find it hard to believe that Yamaha would require a tear down every 5K miles... it's more of an age and use related condition. If someone has been running it recently they may be ok. If not, well then...
Do the test. If your cylinder leakdown test is negative, I'd be inclined to leave it be as long as you: 1. know the sounds RDs make/not supposed to make 2. can read plugs 3. don't have any issues with pinging, knocking, bucking etc, with all else being equal- timing, carbs, etc.
I'm a big fan of preventative maintenance and catching smaller problems to prevent big ones, but remember there are also risks to tearing apart 40 year old motorcycle engines- studs and bolts break in the case, cases can crack when trying to split them and thats not to mention trying to pull items off the end of the crank.
The RDs are incredibly fun bikes to ride, I've owned two, so hopefully you get her back on the road soon regardless of what you decide to do.
Best of luck!
