When I bought my 2001 PT425, I asked about the differences between the 422 and 425 (which may be quite different today VS 15 years ago). They said they would NOT recommend a 60" mower deck for a 422. They said the 425 had enough HP to spin the big deck and keep it spinning in thicker grass. So, I'd say the larger mower deck would require more HP than the 422. You may want to ask PT about this.
As for the single ram steering, don't think that would affect the mower that much as its on its own wheels, except if you bump it out of float. There have been reports of the single steering rams ripping out, so that needs investigation.
You'll have to do some research about shaft lengths, diameters, hydraulic motor mounts, etc... if you decide to change motors.
If both cylinders failed a leak down test that is very odd. Leak down means rings and/or valves. More likely valves, I would think. A valve job shouldn't cost that much and could probably be done without pulling the engine. If it was me, I'd do a valve job, then a compression test. If OK, then OK. If not, you'll have to pull the motor to do a complete rebuild. I wouldn't think a valve job would cost that much.
As for Kohler VS other.... I only have experience with my Kohler. Mine started at 2 degrees F this morning with no assistance, starting fluid, pre-heating, etc... (knock on wood). It has about 600 hours, I think. Oil changes every 50 hours (once a year). Always used Mobil1 since first oil change. Just changed the spark plugs for the first time last year. Same for air filter. I just blow it out when needed.
If the Subarus are not starting in cold weather, I'd think most likely the starter is not capable of spinning it fast enough, or the spark is not hot enough.
Has anyone tried starting a Subaru in cold weather with a booster pack or jumper cables from a car or truck, and one shot of starting fluid before the first crank? It might be worth a try.
