You may have issues with size, I can't remember for sure what the size difference is, however the biggest Issue I forsee is the decompression valve.
On the G4 powered machines, the decompression line attaches to the carby, however on the G3 machines it attaches to a mettal nipple coming from the inlet and not the carby.
So if you pop the G3 across, you won't be able to link up the decompressor.
To overcome this, you could remove the decompressor and block the hole with an old spark plug, or you could transfer the cylinder, piston and rings from the VC-160 to the VC-Mustang along with the carby.
