Without wanting to sound overly negative, it's probably not worth your time and effort to get that thing back into a useable condition.
To "unstick" everything you'll virtually need to lower the whole bike into a vat of PB Blaster, and leave it there for a year, and when it comes out it'll still be a clagged T500 (not a particularly collectable bike, even in "mint" condition) in need of a heap of parts, most of which will be hard to find in good condition.
Is there a "bitten off more than you can chew" smiley? Best of luck mate, if you're serious about returning that thing from the long dead, you're really, really gonna need it........