For simplicity - EFI Robin.
++1.
There are two individuals on this board who have put the 28hp EFI engine in their PT422.
KMA:
http://www.tractorbynet.com/forums/power-trac/202389-pt422-engine-swap-completed-pics.html
Vacuous One:
http://www.tractorbynet.com/forums/...engine-shaking-sputtering-10.html#post2170245
It isn't as overpowered as it sounds: the 28HP is the same engine as the 25 HP engine except for the carburator is replaced with the EFI throttle body. The difference is that the 28 HP is quoted for a higher engine rpm then the 25 HP. Terry from powe rtrac has confirms that the PT422 pumps would work with a 25 HP engine.
One suggestion: if you have the square exhaust manifold that power trac cursed some of us with, then throw it away and find a stock robin exhaust manifold that can be adapted to your installation. The Power Trac exhaust manifold is really bad, and tends to restrict flow and trap heat in the engine compartment.
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It is too bad that Power Trac didn't set up their engine compartments to accept vertical v-twin engines. The vertical engines seem to sell for about $500 less then horizontal versions, possibly because of economy of scale. It seems that just about every lawn tractor or zero turn mower out there uses vertical engines. Some of those mowers are cheaper then just a horizontal engine.
I am thinking vertical engine with a clutch/pulley attached, and a vee belt or poly belt connecting to a vertical stack-up of hydraulic pumps. I think it would fit better, give better access in the engine compartment, and give you a clutch to make engine starting easier.
JJ, can the hydraulic pumps be oriented vertically?
Of course, trying to retrofit such an arrangement would require replacing all of the hydraulic hoses and fittings, since their existing lengths would be wrong.