you say it wont start, there could be several things wrong. do you have a new glow plug in it? is your glow plug igniter charged? does the engine have any compression (turn the flywheel with your fingers, if it gets easy to turn, then gets quite hard to turn then you have good compression, if it is easy to turn all the way around the piston/sleeve is worn out)? are you using fresh fuel?

the slow running of the engine, slower than your friends who has the same motor may be because the engine is out of tune and needs to be retuned. consult the manual that came with it and find the stock carburetor needle settings. set the needles like it shows. once you get the car started on these settings, it may be really slow, but drive it around for a few minutes to get it to running temp. bring it in and lean the high-speed needle in 1/8 of a turn at a time until you see good power and temperatures (210-260 degrees farenheight is ideal for most engines). now bring the car in and pinch the fuel line. the motor should rev some and die within 3-4 seconds. if not the low speed needle is out of tune. if it takes a long time to die, it is too rich. if it dies instantly, it is too lean. adjust 1/8 turns on it as well until it takes 3-4 seconds to die. once you have this done the motor should run alot better.