Just to throw a monkey wrench at you one more time....
Go to the for sale section, and look for Methods "wife kit" sale. It's a slow version of the 9 continent motor, and ideal for future upgrade to a much more powerfull ebike. The Mac, while a great motor, is not so good for pushing 3000w into for a dirtbike.
The kit is very good, complete with a CA at a very very very good price. Get the rear motor of course. At 12s lipo, you'd see about 18-19 mph top speed on the flat, and have ability to climb hills to 15% grades with pedaling, 10% is a piece of cake no pedal.
But later on, Mhahhahhahaha
, Put a lyens 12 fet 40 amp controller on it, and go to 18s lipo. Now you have a very nice setup for trail riding. and still can ride street fine. The only compromise would be an enduro type tire tread.
Seriously, it's a good choice. 3 of 6 ebikes I own are running the slow winding 9c motor for good reasons.
You want torque, feed a slow wind 9c motor 2000w. It'll plow deep sand, through snow, up hills, beautifully.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.