So can I add another mount and use it as the main mounting point in addition to using the 4 holes at the bottom, or should I avoid using those holes at all?Those 4 tapped holes on the bottom were not designed as engine mounts.
That engine mounts with the through bolts that run on the crankshaft axis.
Use the cross bolts at the rear, and if the top bolt on the crankcase is too much trouble to reach with bracket (empty bolt hole in pic below) then use a bracket on a head bolt or two to arrest the engine torque.
You can right click on that picture & open it full size for more detail.
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I don't know of any mountain bike frame that will fit the 125cc lifan. The engine is like 18" long. Here's my build thread if you want to take a look at what I've got so far: http://motorbicycling.com/showthread.php?t=61893. Also, going from a stock chinagirl to a monster like the lifan seems like a big jump in terms of both engine power and the expertise required to put one together. If you want to build a motorized mountain bike, the 79cc or 212cc predator engines might be a better choice, and I have experience with them as well. Heck, I've got a brand new 212cc sitting in my garage that I'd sell you for less than I bought it for just to get it out of there. If you're wondering what you can do with a 79cc predator, here's my current bike. It's an old trek 820 frame, does about 40 on level ground, and still has the power to make it up all but the steepest hills without pedal assist.
