The only thing you can do is put a bigger piston into it. If you still have the stock bore go to a 416 or 426. They will cost about the same. I got my piston and gaskets for 150. Then while you are at it put a cam into it. I know you are tight on money but do what you can do.
2000 Honda 400ex, 426 JE piston, Hotcams Stage 2 cam, Ported head, Uni Air filter,WB Rev box White Brothers Pro-Meg exhaust, TCS ZPS fronts, TCS rebuilt ZPS rear, IMS desert tank. IMS Bumper, Tinner airscoops, AC Propegs, Edelbrock Carb, PEP stabilizer, Durablue X-33 axle, Houser +2 a-arms. Maxxis razer rears. 15/36 gearing
AIM naterbwillard