The Valve O rings are the same for tank orings, those are the easy ones. The valve hole should be fine since you are using the original bolt, that will allow you to adjust velocity.
PPS is prob the most common source for parts including the cup seal.
The exc setup is also fine.
You should be able to use a phantom grip body to get a .45 or any other M16 replacement I think Pearce makes one too.
72872 - AR-15 Grip Adapter, 1911 Style