I have managed to source a second hand 11.5HP Briggs. It is out of a Craftsman I think so I will need to chase down some parts, muffler, tank etc. Tank was frame mounted and muffler is one of those front of chassis exit jobs. I will post some pics once I find these bits. I have yet to start it so I have taken a bit of a gamble. I have simply spun the motor over by hand via the pulley as there is only electric start.

Out of curiosity, would you remove the head or the sump to just have a look see or simply wind it over and see what happens?

It's a 28D707 made in 94.