Just my opinion but I don't think that's such a great deal I'm not a big fan of the 661 but anyway I sell lots of parts on eBay and here are some of the prices so you can at least get an idea of what it will take to fix.
Is it a M-Tronic ? If so dose it still run? You have to be careful dropping or running over will damage the circutry in the carb the only way to fix is to replace the carb and they are not cheap $200.00 for a new one and $75.00 used, New Tanks are 200 I was getting 125-150 for a used tank, case halfs I would usually get 100 each, I was getting 50-60 for used 1/2 wraps and 125-150 for used full wraps, used crankshafts would bring 150 and up, used starters were 80 and up as you can see parts are high as hell but whole running saws only bring 700-1200 depending on condition. Those are eBay prices don't forget to account for that hope this helps if you have any questions dont hesitate to ask I'll try to help