We modified a cabinet to put a 15-91 in. The motor sticks up too far when put "away" for the support flaps to sit flat. We thinned out the backside of that part so it would go all the way down.
We also had to move a support bracket so the flywheel would clear.
You'll do best if you start with a Singer treadle cabinet, otherwise you may be in for doing a lot of modifications to get the machine to fit.