yamaha bw spy 50 1998 help!

[loudmatter]

Hi guys in need of some desperate help. I brought a second hand Yamaha bw spy 50 as a non runner,
managed to get it going but just barely. I have had new piston and piston rings, New fuel lines, unrestricted cdi, New spark plug, New variator and 6.5gr rollers with new centrifugal spring. The bike already had a Leo Vince tt exhaust fitted and it has a #98 main jet, so would this be the jet that came from the factory? or did it come with a lower main jet?
If I have the standard air filter box it seems restricted at WOT and tends to slow down so I am having to keep it at 3/4 throttle to stay near 30mph but if I have a k&n performance air filter it hits 35mph but then the engine stops after WOT for a set period of time so plz plz plz help me out guys what can I do to have the bike running consistant and above 30mph.
Thanks in advance.