Glad I could help!

The easiest way in my opinion would be to locate a new identical valve, and swap the movable parts - unless something is wrong with the body of the valve. The plastic piece you mention is a sleeve used for Pex connections, but is mostly used by plumbers and is quite permanent. You can always cut the plastic tubing below it, but then you have other problems to solve... If that connection doesn't leak, I'd leave it alone, and swap the inside of the valve with new parts.