Just a simple frame made out of scrap 50x50 square tube, a bit of angle iron and 4 castor wheels - but it sure beats tryng to lift them! This one was made to suit a rod change gearbox, although with a bit of thought it shouldn't be too hard to modify if needed.
The front of the gearbox sits on a piece of 50x50x5 angle iron.
And at the back the diff housing just rests on the square tube.
Note that the engine is not bolted in! While it can't fall out, it's not really child proof or meant for long trips - more just a short push under the bench or out of the way. Brackets can easily be added though if you feel the need.
These things are always best made out of materials you already have, so there is no need to stick to my design if you don't have the right size steel. Here's a quick layout to get started with, but there is nothing really critical in the dimensions.
I have allowed a good 5mm clearance all round the base of the gearbox, and all dimensions are measured inside to inside. If you want to close this up for a neater fit I'd suggest trying everything before welding. I also have the front wheels sticking out twice as far as the back ones because the engine is front heavy.