The back plate on an LA engine is only a few bucks, and it is a very easy repair.

If the part that seals up the back of the motor is solid, and your repair is just holding the needle valve on, then you should be fine. If it comes loose, you can just zip-tie the needle valve to the motor mount until you can get a new backplate.

Good luck

Edit:

http://www3.towerhobbies.com/cgi-bin...?&I=LXPV83&P=Z

OK, maybe more than a few bucks, but still worth fixing.