Hello Doodlyde, welcome to the forum
I looked at your bike online and it looks like you will have plenty of room for the center frame kit. But it is hard to be sure by a photo. At least 12.5 inches running along the seat tube from the top of the bottom bracket to the bottom of the top tube.Tubing size from 1-1.5inches on the both the seat and down tube. Plus or minus 70deg. above the bottom bracket between the down tube and the seat tube. Inner distance between pedals needs to be greater than 4.5 inches in width to clear the 4.5inch wide motor. If you have bigger tubing there is plenty of ways to get around that.
1. Yes, Sick Bike Parts has a shifter kit
Sick Bike Parts LLC - Performance Parts for Your Motorized Bicycle
2. No, as far as I know there is no motors specifically made for mountain bikes. If you use the shifter kit you are stuck (for the time being) with the Chinese center frame 2-stroke. I would go with the bigger motor instead of the 50cc, it will give you more torque.