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I have heard that FF3j is the biggest pain as far as nes repros go, I have yet to find a single, clear, guide on making on, the best I have managed is a picture of a finished cart's board. However I have an idea, and wanna know what anyone thinks.

If I were to have an old nes game, say Gyromite, that had a famicom converter built into it, and I were to remove the famicom gyromite cart. If I were to have an original japanese ff3 famicom cart and replaced gyromite with it, and replaced ff3's mask roms with the translated game on eproms, would it work with minimum modification, or would it require just as much work as using a SMB2 NES cart?

Thanks in Advance.

That would be less modification work. You would just have to follow the usual steps for rewiring a PRG-ROM of that size, without having to add a bunch of stuff to the cart.