how you figure the mile per gallon about a small two horse power lawn mower engine put on a 27 " bike?

If you already installed it and it works then you divide the number of miles you went by the number of gallons put in.

example: 2 gallons went 100 miles 100 divided by 2 = 50 so you got 50mpg. If same 2 gallons went 20 miles it would be 10mpg....

You cant figure it out using just the information you have provided, there is too many variables. eg. Efficiency of the motors to start with, drive train losses, air resistance of the rider.You really need to build it and then do some empirical tests. I think you will end up with very good efficiency in terms of amount of fuel per distance traveled, but it wont be very fast.