I wouldn't suggest using the foot peg mounting holes on the bottom of the engine as a mounting point, you'll have to make a rear and top mount as those engines have two mounting points... Drill the mounts out on the engine case to 3/8" as the factory holes are off size and I have yet to find a metric bolt that isn't sloppy in the holes....You'll also have to use a spring loaded chain tensioner or make it so the wheel can be slide back to adjust the chain since these engines are mounted solid with no adjustment....
Most of the 125's are about an inch longer than a 110 so make sure you have enough room between the rear tire and front down tubes...
There is alot of fabricating, measuring, measuring and more measuring to get the engine where it needs to be so the chain runs true and everything has clearance....
I've done a couple 4 stroke conversions on 49cc 2stroke weedwacker engine style pocket bikes, it's not a quick conversion if you want it done right..
I suggested using a mb engine since you told me you didn't know much about engines or how to work on them when I inquired about purchasing that briggs and the extra parts you were trying to sell..