The spring is available at the AGCO dealer, about $17. The spring was deleted at the factory on the later tractors as it was found to not be required. Better bet is to buy a cheap long low tension spring (about $2.50 at the AGCO dealer) and fit it to the governer rod hole.
BTW, I had a C that did the same thing. I eventually swapped it. The owner reports the same problem, but it goes away when FULLY warmed (about 20 minutes).
I'll be interested in how you fixed it.