To make a gun "Lipo Ready", perform the following. Gun + Lipo enough said....
A mosfet can help save additional ware on the trigger contacts but is not necessary. I’d place it in the nice to have category. I'm not familiar with the stock parts on a CYMA but changing from plastic to metal bushings would be a good investment. For everything else, if there is nothing wrong with it, leave it alone. If something breaks then just replace it, after all it's a CYMA.
If you want a High ROF then yes, you can invest in a new high speed motor, high speed gears and ect...but you could probably buy another CYMA at that point.