When thinking MX....I'd think more about suspension & chassis before any motor work...well maybe pipes, filter, & tires.
Shorten your swingarm -2 & get you some good A-arms & shocks setup for your weight & MX. With even 7 G's you could easily get this done, along with gussetting & powder coating the frame. Hell,...get a stock shee with those mods & no motor work and you'd still be better off. IMHO...