Do you have a purpose built garage for your mower or do you just have shed that you store your mower in? When I borrow my neighbors mower I normally put it one of the small outbuildings.
What do you think is the best engine on the market for walk behind mowers? even though I don't own one of my own I have used several with Briggs and Stratton motors and thought they were quite good.
When buying a second hand engine can you use any engine in any similar mower even if it is a totally different make and model? I'm aware that some modifications would have to be made if it could.