--That is an amazing undertaking, and a smart guy to build a processor from scratch.
--He obviously has a lot more free time and ambition than I have.
--Troubleshooting shouldn't be that bad as long as he has a diagram. He obviously understands the principles of operation, so he would just need to put a scope on the correct test point and make sure signals are present. The behavior will tip you off as to what's wrong. Many years ago when I took electronics, we had a donated DEC PDP 11/70. Our instructor would sabotage it, and armed with blueprints and a scope, we'd have to find the faulty component or failed wiring.
--Looking at the MIPs on the processor comparison, it's obvious the MOS6502 was the greatest bang for the buck in its day. Half the transistors and clock speed and still as fast as its competitors.
That is all :)