You can also get isopropyl alcohol which does much the same job. Both quite common household or workshop products. In cars we used to put about a cup full of meths i and it would generally stop the water getting into the carb and blocking the jets. If it wasn't enough, you added a bit more. It's not so much how much fuel there is, but how much water you want to absorb. If you've already cleaned the tank it should only have a trickle of water left at most so probably a half or quarter cup would do. If you put a lot in the engine runs a bit lean and weak. It's alcohol though and burns cool.. It might make the bike a bit gutless but it won't over heat or do anything bad.

I've always just used methylated spirits.

Regarding changing to a higher quality oil after a life of lower quality: Back in 1970 (newly married, and little money) I traded an old farm truck for a early 60s Corvair. I was driving it on the freeway, and a huge bang sound and no drive power. Pulled over, popped the hood, and found a rod poking put of the crankcase. I had recently changed oil. My go to oil was Rotella T, a high detergentcy oil. I later found out that the previous owner had been using a very basic oil with few detergent additives. My take was the higher detergentcy oil loosed some chunks of deposits that then clogged up an oil passage to the failed rod. This is all anecdotal, and I could be way off base, but I would be wary of changing types of oil on an old vehicle. That's my story and I'm sticking to it. tom

Is your choke lever mounted on the left handlebar, under the switches? if it is the thumb-style lever I recently fixed one of those. See the link: there's a special washer setup that gives it friction to stay in the on position when required. Parts 13/14 did the trick. https://www.ronniesmailorder.com/oemparts/a/yam/50038e6ff870021f60a0c214/handle-switch-lever