Yeah this decision has been haunting me for the past couple days. How much do you think an engine revuild would run me if it gets over my head? This is hypothetically speaking of course since I don't know the true condition of the bike. I got the xs650 for its reliability and ease of maintenance and dialing in, but I feel that it just won't be worth as much as that thunderbolt. Worse comes to worse, I can always get the bsa and grab another xs650 while I dump my time and money into the bsa... this decision is a lot harder than i thought it was going to be as I am attached to my xs650 even though I didn't get to ride it all that much.