Re: is it worth the potential trouble?
Too hard to tell without some close up, detailed photos of that $100. bike. Things like engine and chain tensioner mounts, rear sprocket installation and the overall condition of the bike would all be factors you'll need to consider.
First question would be, does it run?
Have you ridden the bike?
If the seller is only asking a hundred for it you could well end up with a hundred dollars worth of questionable spare parts. Nevertheless the bike could provide you with some training and experience on what not to do with your next project.
Provide us with some pictures and we'll be able to offer more advice.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"