josh:
i have just finished building a big bore rm144. as sawblade says, eric gorr is the person to work with. yes, you will get much more power.
however, if your engine has three years on, it will probably need new bearings and seals in the bottom end- it will definately need them if you are going with a powerhouse 144 bigbore.
also note, the big bore uses a 58mm piston vs the 54mm stock. you will have to slightly file/grind down the cases to make room for the new piston- it is best to do this when you have the bottom end apart, so youve got a good excuse to change them bearings!
either way, if its never had a new piston in three years, put one in before you ride the bike again- its probably worn to the point of blowing up!