In 1997 I attended the Massey Ferguson 150th Anniversary Celebration in Milton, Ontario. There I met Brian Mc Govern who had researched the Massey archives for the GP serial numbers versus years produced. This is what I recorded in my notes.
1930 - 300001 to 301417,
1931 - 301418 to 302851,
1932 - 302852 to 302979, these first three years production all used the Hercules OOC flat head engine.
Starting in 1935 the GP was equipped with a valve in head engine, Hercules OHC, serial numbers 303001 to 303400. Apparently these were all produced in 1935.
So this would make your tractor a 1930, the one hundred eighty second made.
It is interesting you have the mechanical lift option on your tractor. It would be fun to play around with that device.
In England David Parfitt was maintaining a huge registry on the site steel-wheels .net which included GPs. However this has been recently taken down, google it for the whole story. There is a member on here with registry information,maybe he will chime in.
The early tractors should be grey with dark red wheels and the later model green with red wheels and yellow decals, the same green as used on the Challenger and Pacer.
I may be able to help on a magneto and possibly radiator. Give me a call on five four one 570 two one one three if interested.
Alan Painter, Brownsville,Oregon