LOL! I guess I didn't make myself clear. That was part two of my question, would the 18.5 fit. It looks like it will. Looks like they are the same frame. The deck and frame rusted out on my old one. The engine was strong last year though. Looks like I need to do more wrenching and less typing and get that engine out, and do a once over like you said.
It depends upon the hood.
Some will need to be cut away a bit to prevent fouling on the engine.
The bolt mount spacings are different on the V twin so you will have to drill some new holes.
So you can measuer out , draw up then make a cardboard template from the drawings on either Briggs web page but I find the drawings easier to find on Small Engine Warehouse or Small Engine Suppliers web pages .
Or go the lazy way ( like me ) and place the new engine so that the pulley is in the same place ( thus uses the same belts ) then mark the position of the MOWERS bolt holes on the new engine.
Thus when you take the motor out you can see how far away the new holes need to be and then decide if you will use any of the old holes.