Telmo Moya has powered his C64 with ARM cpu as direct socket replacement. Now we do not have C64 with pure ARM power as he developed dual core cpu replacement emulating 6510 and Z80.
Why Z80 someone may ask? In 80’s Commodore developed a CP/M-cartridge that contained a Z80 so C64 could benefit of CP/M software. We also need to mention C128 that had Z80 on board with CP/M BIOS.
Both COres 6510 and Z80 are written in C.
Check Telmo website telmomoya.blogspot.rs and enjoy Dual software C64 core video: