Not an expert by any means, but a couple of thoughts occur to me.
Some times when a conscientious owner is placing their beloved motorcycle into storage for an extended period of time, they'll pull the spark plugs and put "fogging oil" in the cylinders to keep the cylinders from rusting and also keep the rings from sticking to the pistons so that they function properly. You may be seeing an overabundant application of this fogging oil escaping through the exhaust system?
The more likely cause of the symptoms you're describing is that the bike has sat for 20 some years, and the rings *aren't* doing their job, which results in crankcase oil getting into the combustion chamber where it fouls the plugs and escapes through the exhaust. Sitting for a long time is hard on things. I've got an old tractor that my grandpa bought new in the 40s. He passed away years ago, and my dad got the old tractor. It was old and small even when I was a kid. Dad used it for a few years, then it sat in the corner of the shed for 25(?) years. I finally inherited it a few years ago, and it ran but had excessive blue smoke. I used it that way around my small property for a couple years then a couple years back I needed some decorative stone for a landscaping project. I didn't have a car that would pull my landscape trailer the 10 miles into town for that rock, so decided to use the old tractor on some hilly back roads. Well, that old tractor has a really fast road gear--fast enough that it was working really hard pulling a loaded trailer. I had to shift down a gear at times to make the hills. Anyway, after that the engine doesn't smoke near as bad as it did. I figure working the tractor must have freed up the rings.
I would guess that getting this to run properly may require a tear-down and manually freeing up the rings or possibly replacing them. I don't have that kind of experience, so I'm only guessing. You may have some luck with putting some diesel or Marvel Mystery Oil (this is an actual product, if you're not familiar) down the plug holes to free up the rings. Again, this is beyond my experience so maybe somebody else will have better advice.
You've got quite an ambitious task if you're going to be bringing an engine that has sat for 20 years back into service. Aside from the oil you're seeing, you should be checking the air filters. Very likely they've disintegrated and may be getting sucked into the carbs. The carbs need cleaning also, no doubt. . . . Here's
a post where I describe the process for cleaning the idle circuit. You'll need to pay particular attention to the idle circuit, but the rest of the carbs will need attention as well. Check the diaphragms for holes as bike may run but not make power if they are compromised.
Coolant will need to be drained and flushed, and new coolant added. . . . Brakes also, will need flushing. There will likely be hoses that need replacement. Be aware that all these bits and pieces add up--this will be a labor of love if you're successful. It might make more sense to find another bike that runs better especially if you're just learning to ride. This bike will not be as safe as a bike that runs right, and you'll need it to work as well as possible to learn easily. Speaking as a forum member, I enjoy giving advice and there are lots of other members that are very knowledgeable and helpful. Just be aware that this will cost you in terms of time and money, and may well be very frustrating. Not sure how much of this stuff you've done before, so don't mean to tell you stuff you already know. . . . Good luck with whatever you decide.