There's lots of different ways you could make this , it comes down to personal preference, I would not make a mower from a deck because I don't want to replace a V belt and replace spindle bearings when you can just mount 2 smaller engines with blades on the crankshafts.
If I went the way of using a ride on deck it would probably cost over $500. for time and parts.
I've used a Deutscher walk behind mower in the wet and it worked well but don't know how muddy his property is.
If I made something like this, I would make up a frame and use 2 engines but why spend many hours making a mower up when you can just tow two 19 inch mowers behind the motor bike , you could just bolt two mower bases together.
There was a gang mower in Silvan for about $300 for months on Gumtree ,that would have suited but then again it's probably not going to get into corners but towing two 19 inch mowers the bike can be reversed.
