speedfight running problem?

i have a speedfight 2 100cc 1998 model i was rideing it and it back fired and cut out and didnt start again i was told if its not the cdi it could be the crank seal and bearings so i changed them and also changed the piston head and barral with a 2002 model and changed the carb for a 2002 model and had 2 change the throttle cable thats all done and now the bike starts but it runs worse then a 50 it pulls away so solwley but its fine when you get to about 25 mph it was not like this befor i had any work done to it and all the work was done by my dad who has been a mechanic for 30 years so im sure nothing has been put on wrong but i could be worng i have been told that it sounds like the timing is out but i have no idea how to adjust it can any 1 help me

Thanks :)