You can work out the kinks wonderfully
How? Daytona Guns
These guys make custom Steel internals for GBBR it does come at a hefty cost depending on how big or how many parts you need. And sometimes you would have to send your internals to be custom machined so they know what parts you are looking for. They even have some internal upgrades to fix the old Maruzen MP5K from crapping up
Heres an example of their work
You can wait perhaps a year or two for companies to overcome their learning period for GBBR's and wait for them to get better. AGM is the closest clone and cheap.
There are also drop in kit is available from RA-Tech and many more things to come