Springs were available from Case about 5 years ago, not sure if discontinued now.
Tractors prior s/n 5365687: gov control spring p/n VT3280, cross-ref pn A30455.
Foot throttle lever return spring same on all VA series: p/n VT2435, cross-ref p/n A30339.
If not avail from Case or salvage yard and the gov otherwise works properly, the gov control spring can be substituted trial & error with a similar generic extension spring.
The foot throttle lever spring is kind of unique, some jury rigging reqd.