The HP doesn't have to match exactly.
Best to stay with the same # of cylinders to avoid other issues that might arise, such as muffler...

IF you posted the Model & Type/Spec of your blown engine, maybe someone could point you in the right direction.

Assuming your engine is a Briggs and Stratton, you will need to replace it with the same vertical crankshaft engine that has the same shaft dimensions, etc. You could "possibly" do a twin cylinder swap if it's a vert. shaft engine and there's room in the engine bay but the you will be making A LOT of modifications. Probably wouldn't be worth it on that tractor. See the link for a 17.5HP Briggs single cylinder engine, around $500 plus shipping. There's also the similar Small Engine Warehouse. I did a motor changeout using SEW as my source. Went without a hitch and the replacement motor has both pull start and electric start... nice backuo feature when battery goes dead.

Here is a link that might be useful: 17.5 HP Briggs vert. engine