I did the opposite, I lowered one 2.5" for my 7 yr old son. I machined a new swing arm connectiing rod. Stock was 6.9" between centers I made mine 6.125". I use broze bushings instead of the ball joint like stock, but I expect it to last a couple years. I have machine tools so it was easy for me, but I would think you could make one good enough with hand tools. Let me know if you want some help.
Then I pulled the fork tubes up into the triple clamp so that they stuck out 1.1". He loves it. If I put my 220lb. booty on it the back tire rubs! I would think you could sink the fork tubes 25-50% down into the top tripple clamp if you use a non permanent loc-tite between the clamp and the fork tube.
Jim