rotax 503 question

The best way for you to come out is to bolt the sled motor into your UL and the
UL motor into your sled, sell the sled and have a better and longer lasting
engine in your UL

Did the pusher use belt or redrive?

I had the same situation switching an engine from a MiniMax with B Box to a
Quicksilver with belt drive. It turned out to be no problem since the B Box
reverses output direction. I was able to use the same prop.