Before people say, You can play around with engine. Do you actually know how to pull it apart and do you have the right tools? Take it to a mechanic. They are cheap, reliable and they know what they are doing and have the tools to do it!
Recommend :
Epping Bikes
9401 1377 - Pretty sure thats their number. Dont hold me to it though, give it a ring tomorrow morning and tell them whats wrong. They are really great people.
If the bike needs an engine rebuild, It will prob cost you 200-300. You may aswell just get another ****** MXB off e-bay. They go for the same price. Lol'd
P.S. PJ i love you