Yes, you need to pick the diameter of the rear tires.
It's A direct drive, so it doesn't have any internal gear ratios.
I just switched to A Ko Propo pds-949 servo, and it's A very good one, one of the best out there.
Before that one I was using A standard size Hitec digital servo, but it didn't center at all, so I bought the ko.
A standard size servo fits in the 12M without any problem.
The Futaba S9550 is A "low profile" servo, so it's quite thin, but for the rest it looks like A standard size servo, so I guess there shouldn't be any problems.
If you want A new servo and want to stick with Futaba you should get A S9650 (digital), used by A lot of people for 12th cars.