In my testing of the 100w PowerSun, I found that the UVB output decreased quickly after burn-in and then stabilized at a relatively low (disappointing) output. Unless I received a dud (which is possible) I think the PowerSun would be OK in a smaller enclosure sitting at about 8-10'' above the main basking area. Depending on your surface temp readings when the MVB is at that distance, you may need to use one of your basking bulbs alongside it, but if you're still getting readings around 100, then that should be fine.
As mentioned, you can revisit this once you are ready to upgrade your enclosure size. I recommend doing this sooner rather than later because having more space will give you more leeway for building temperature and UV gradients as well as provide more enrichment through basking rocks and climbing vines etc. If an upgrade is not within budget, that's OK for now but he will outgrow 20g by around 6-8 months old or so depending on growth rate.
From there, I would move to a T5 using one of the bulbs and fixtures suggested in earlier posts. I like the Arcadia 12%, but the ReptiSun 10.0 is a good option as well. The Sunblaster fixture is good if you want to hang it from something - either the top of an enclosure or a stand since these are designed as grow lights and have clips on top. The ReptiSun hood is also a very good fixture, but is designed to sit flat on top of an enclosure and isn't as straightforward to suspend as a grow light fixture is. As mentioned, the ReptiSun hood comes with a 5.0 bulb which should be replaced with a stronger version in order to provide effective levels of UVB at a reasonable distance in a larger setup.