OP:
Only way I can see that the screw could have bent the valve is if it got caught between the head of the valve and cylinder head when the valve opened. What kind of shape is the head in?
I think you need more than just a valve. Seriously......
Buy the valve from this guy on ebay:
http://stores.ebay.com/The-Mans-One-...=p4634.c0.m322
There are lots of cheap carbs on ebay. Several members here have been buying them with good results. I have not bought any of them yet. I have lots of motors, and I just rebuild the carbs I have.