There's the mostly 6502 compatible 16-bit WDC 65C816 that'd probably be a much better, less painfully minimalist starting point.
Apart from a MMU, the biggest hurdle I think for modern dev on the original 6502 is the lack of multiply/divide and the pain of having to either forgo modern levels of stack use or emulate a bigger stack.
Apart from a MMU, the biggest hurdle I think for modern dev on the original 6502 is the lack of multiply/divide and the pain of having to either forgo modern levels of stack use or emulate a bigger stack.