Two very easy ways to do that.
The first is to make two saw cuts on the edge of a one foot or longer one by six board. The cuts are the width of the engine case not the outside of the engine beams. diagonal cuts are now made to whittle out the notch for the engine case. Screws or bolts now secure the engine to the board.
A simple block of wood glued or screwed to the main board can now be used to provide a base for the fuel tank slightly elevaded. screws into the side of this block in four corners provide a place to hook rubber bands over the tank to secure it. The throttle rods can be any pushrod wire with an S bend and this slides under the rubberands along side the tank and provide a stabile throttle wire that will not move until you move it.
Finally this board is secured with dry wall screwed to an outdoor bench or substitute.
Another variation of this is to simply bolt a spare engine mount to a board added to the flat on to provide a place to bolt the plastic mount to.
John