Its probablly strong enough but if you turned a 1 by 4 on its side and ran it from the board that angles down at the bottom where it joins with the board right above the castor, horizontal to the boards that go straight down it would ad alot of strenth.
I built one very similar to your design. But it is MUCH larger. I have had everything from a 3.5 Evinrude all the way up to a 250 Yamaha on mine. The casters are great for moving around. Plus you can work on one in the shop and roll it outside for a test run.
That little shelf is PERFECT for a small gas can and a battery. Comes in handy when you bring a mystery motor home for some trouble shooting. Mount a universal control on the side and you have a great test stand and motor storage.
I am thinking about swapping my casters for some of the BIG inflatable off road casters. For even more manueverability.