I tried a larger flywheel. It took about 9 motor revolutions to reach 50% speed, so once I change the gear ratio to 1:1 I'll be getting within spittin' distance of my target. To that end, I ordered a Custom Works Spur to Pinion Adapter (#CW8225) so I can use the same size spur for drive and driven gears.

I also ordered a 1/2" diameter shaft and bearings to support the larger flywheel, and some pillow blocks to make assembly of the test rig easier, plus another identical flywheel to double the inertia.

The flywheel I used is a weldable steel hub for V-pulleys. It has two set screw holes at 90 degrees from each other, which makes it easy to balance by simply attaching weight as necessary to the threaded holes. Of course, that also means it is necessary to balance it! The best part is it only cost $12 on the evil auction site. Here's a picture: