If the day comes when you need to "repower" this machine you will probably try to go the same route you did acqiring this one. You'll look for a bargain tractor that runs well. But if you insist on replacing the engine, 2 cylinders are a bit smoother running than 1 cylinder. 3 are smoother than 2 and so on ad infinatum. Horsepower is horsepower regardless of the number of cylinders. You can probably go a couple of horsepower more than the current engine but trying to "super power" a lawn mower is just like any other vehicle, you put in more engine than it was built for and something else down line has got to give. Other important details are mounting bolt pattern of engine to frame, crankshaft diameter and length, exhaust system components compatibility (either you gotta use your old exhaust or the new exhaust has to fit your chassis. Anything that is mounted to the engine must clear all sheet metal, plastic, or fiberglass of hood or modifications will be needed.

I'd be concerned about the MOWER part. The engine is pretty much secondary.

I would not consider reengining a 15 year old Murray. The cost of the engine will be more than the whole mower is worth. You have certainly got your moneys worth out of this tractor. Run it till it quits then go out and buy another 5-7 year old tractor and hang on to it for 8-10 years. The twin will run smoother, but it will use more fuel, and may be less reliable than a single of the same HP. There is more to go wrong with the twin, and more parasitic losses.
steve

Ok, guess I wasn't too clear, sorry. What I'm saying is, once I retire this tractor, should I replace it with a single or twin cylinder? Once it goes, I don't have the intention of putting a new engine it it. With the light work that it will do, will a single or twin cylinder make a difference? Thanks again, Greg

It really makes no difference when you mow a lawn once a week. IMO HP per HP they don't use more fuel. They are quiter ( but that depends on the muffler), they have less vibration, IMO hold there Torque range longer (two arms working vs one), they are usually made to last longer.

Now the down side. They usually drive the cost of the LT up (meaning the same Tractor with a twin over a single will be few hundred more). Up keep cost more, odd ball air filters, and oil filters. Maybe a tad more gas, but I can't tell the difference between my single over my twin. Repair costs will probably be higher.

IMO bottom line on the lower end models it don't matter, just get what features you like if that includes a twin so be it. Most of us are more concerned with what brand of engine they come with.