rpm's ?

Until you can get your hands on one . . . If you're accustomed to running the tractor and have some sense of what it sounded like before you lost the old governor spring, you can dial it in by ear, but go on the slower/conservative side. If the spring is indeed different, your engine will thank you for cranking the governor high idle screw back rather than trying to remember something like "Never go faster than four notches from the front." That'll keep you out of trouble.

In the end, you can nail it with a tach. There are the optical ones that use the patch of reflective tape that work well. Since the SuperC has a battery/coil ignition, you can also use one of the old-style dwell/tachs that hook onto the primary lead from the coil to the distributor. You can also use a contact wheel tach, but that is a lot less handy as every measurement requires some double-fisted arithmetic to convert your reading to engine speed.

Hope this helps!