You should be able to get one at just about any auto parts store. I believe the breather line is 3/8". If your valve is missing, that is creating a intake leak. The bike should run better when you replace it.
Is the front breather line attached to the manifold?
Hmm, that is very strange. Pull the check valve out of the breather line and start the bike. If you put your finger over the breather line, you should feel a vacuum. If there is no vacuum, then something is screwed up. It made me wonder why the PO removed the valve in the first place. You can run both breather lines to the ground. Just remember to plug the hole in the ac and cap the vacuum port.
Also check the front breather line. Maybe it got clogged up somehow. Check the vacuum port on the manifold, the breather line, and the port on the head.
Jim reference your Blow By problem, Is it possible too much oil has been added to the crankcase on your last oil change. You have a drain plug on your oil tank and a drain plug on your crankcase. If you drain just your oil tank and add oil to factory specs without checking the dipstick your overfilled. If you are overfilled you can use a turkey baster to suck some out, clean breather housing and address bottom of air filter that is probably saturated with oil.
