Get their kit.. You will never regret it.
Here is a tip... Use light touch of Locktite on the mounting bolts. I went for a ride today, intending to rack up another 125 miles or so. After getting started, I noted the bicycle frame and engine were vibrating, shaking much more than normal. I finally stopped before a catastrophic failure occurred. I found one of the bolts mating the shifter kit frame (vertical piece) to the engine mount casting had almost completely vibrated loose and unscrewed itself about eight full turns. That is a lot.
My bike is now at home, where it will receive a full mount bolt check, each one of them. If I can clean out the threads with brake cleaner, I am going to Blue locktite those bolts. I think Green Locktite would be better, it holds the bolt from twisting out, but will not make it nearly impossible to back out the bolt when I want it to back out (Blue Locktite has that characteristic).
Understand that you may have to get to them someday yourself to tighten them down.
My bike has over 500 miles on it. If I can tighten those bolts, maybe tomorrow it will return with over 625 on the odometer.
MikeJ