First quick fix, replacing the hand-truck bearings I never bothered upgrading with more legitimate bearings. The original bearings had deteriorated so much there was about 10 degrees of slop in the back wheel.
Since the new bearings actually have precisely bored I.D.'s, I had to turn down my front axle a little bit to match:
I added an eccentric sprocket on the drive-side swingarm for tensioning the chain:
The sprocket gets clamped in this block, and rotated to apply the desired tension:
It's mounted to the scooter as high as possible, so there is little possibility of it scraping when driving over obstacles:
Finally, I added some grip tape to the deck. Sadly, clear grip tape isn't particularly clear. Next time around I'll probably go with plain black: