95 XCR 600, rebuild, replace engine, or cut my losses?

So i bought this sled this fall, the guy said it ran great last winter, just clean the carbs and you're good to go. Can't beat it for $300 right? Wrong. Had to completely replace the fuel system, there wasn't even a pickup tube in the gas tank. The choke was missing all the littke doo-dads on the end of the cables, and it had no spark. Replaced the stator and CDI box, cleaned the carbs up and started it up, and the engine has a clunk in the bottom end. My question is do I spend $170 on a used crank, 270 on a rebuilt crank, or $380 on a used engine? If i rebuild i will do the top end as well and drill out the pto side oil port. I just can't decide which route to go. The top end kit with pistons, rings, and wrist pins is only $115, and i can have the cylinders replated for $100 if need be. Also for about $100 i can get all the bearings to rebuild my crank but I've never done that before, how hard is it? Thanks for any help you might be able to give

95 XCR 600, rebuild, replace engine, or cut my losses?