18mph version? lol ouch
replace your pipe, you'll see a little improvement. I have a sneaking suspicion you may also have the rare Dellorto 14/9 carburetor? Most Garellis came with a 14/12, in any case you can take a dremel and bore that sucker out to 14/14.
If you're really anti-kitting, you could probably buy someone's old cylinder when they get a kit, or try to find a trashed Garelli that you can scavenge parts from.
other than that, all the usual cheap tricks apply: mill your head to raise compression, find a smaller rear sprocket, research porting your cylinder, etc.