Interesting build, basically ditching the blower part and just using the motor and drive to move a large trailer around. Two things that I thought maybe worth doing. One, add a third caster wheel to help with stability. Two, I am not so sure I would have welded the front case together, having access to the pulley system could allow for other builds off of this in the future.
https://www.homemadetools.net/forum/gas-powered-trailer-dolly-video-87331?homelistings#post188737