Why will my baby not go?

So I recently acquired a second '84 FA50. Had one that was a parts bike - no engine or carb but had all other components. The second was in operating condition topping out at about 18mph. Frankensteined the two bike together (serial numbers differ by only 500) and got her 100% street legal. Drained and refilled the tranny twice, new tubes and tires, and new fuel lines from gas tank to fuel She was awesome!! I was hitting 25mph after fine tuning the carb.

So, I decided to continue to restore her and replaced fuel and vacuum lines from fuel valve to carb. She wouldn't even start - I had forgot to prime the carb - so I redid the line from fuel valve to carb and tuned carb again. Fired her up and she sounded better than ever so I decided to take her out only, when I sat down and gave her throttle, she would barely move. Pulled outta my drive and rode downhill but it just wasn't the same.

Couldn't make it up the hill and had to walk her home so I put her back on the kickstand and throttled up and the rpms STILL sound better than ever! Am I looking at new clutch time? The running bike has 4900 miles on it and the parts bike had 3200. If it IS the clutch, should I try to rob one from the parts bike or would it just be best to get a new clutch and install into the running bike?