i have 2 ICS M4s, both configured very differently from stock. They aren't RS parts compatible, or of exacting RS dimensions. It does take some creative modifying and/or parts knowledge to make changes happen. If you are a tinkerer with patience, you might even enjoy the process.
Otherwise, get a G&P for the externals, but be prepared to go into the version 2 gearbox and swap out weak components that are likely to fail with routine use.
That said, the ICS split design gearbox does simplify spring changes
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