the engines off of lawnmowers kinda rely on the blade to act as a flywheel, without the extra rotating mass they're kinda gutless and die easy... I've thought of just making a round steel plate for a flywheel and balancing it, but haven't gotten that far...

the oil system is just a slinger powered off of the crankshaft or camshaft, I don't think you'll be able to make it a horizontal engine for that reason alone. as far as getting the crankcase to split, all that's holding it together is the bolts around the bottom, and the rust around the output shaft at the lower crank bearing. if you really want to take it apart, have the engine upside down, take emery cloth and clean off the crankshaft at the crank bearing seal, get all the bolts out holding the bottom cover on, carefully pull the cover off, try not to shake much around as I did when I pulled apart the 3.5 briggs I took apart in high school, the cam and crank gears weren't marked, so I don't think I had them in right...

the other way to do it is just leave it horizontal and run a belt drive, a V belt can be turned 90 degrees without a problem...