DNA 50 Problems???

I have recently bought a 2001 DNA 50, when going to collect the bike owner informed me that it needed a water pump and that its been stood for 2-3 years, once servicing the bike ran ok but I wasn't really satisfied, a few days later the bike suddenly started to bog limiting me to 5000rpm and only moving a few feet with full throttle. I've bought a new cdi/coil unit thinking it could be that but it wasn't. I then had to strip the whole engine apart to put the water pump impellor in. Once rebuilding the bike its was no different. It has a 72 jet in with the unknown head (owner said was a 70cc). Ive ordered a new air filter too. Can anyone give me ideas what it could be???