1.Turbo board with MC68060/90-100MHz, + USB + LAN + IDE with DMA + low latency and fast RAM for A4000 and A1200 would we outstanding,
2.Maybe enhanced motherboards replacement, but with USB, LAN, fast FlashROM for kickstart and MC68030, or MC68060.
Problems with Custom Chips could be omitted by placing only sockets for it.
In that case customer need to get chips on his own or just take chips from old motherboard. Problem could be also to get DB23 connectors.