Go to smallenginewarehouse.com, click on repower old equipment, fill in your machine info and see what they have to offer.
Zero turn mowers are dusty by design and although SEW doesn't show the B&S engines that have Cyclonic Air Filtration in the JD Z425 repower page, I strongly recommend that you consider one of these models as the air filter does a much better job of filtering the incoming air and the filter is easily cleaned by tapping it against a hard object shaking out the dirt.
If your machine still has a manual choke lever, I recommend that you use a replacement engine that has manual choke along with electric start, a 16 AMP charging system and fuel pump. It is a good idea to know what the crankshaft dimensions on the old engine before you go looking for a repower engine. 1" X 3 5/32" is what SEW shows for a JD Z425, but sometimes this info is not correct.
The engines with Cyclonic Air Filtration have a large bulging housing that holds the air filter. If your machine has the vertical clearance for this, I strongly recommend that you go with one. You can look thru the SEW B&S vertical twin engines and find exactly what you need as a repower engine. There are other sights that have specific repower info so check them all out and educate yourself so you can make a good decision.
Bear in mind that the electric clutch and possibly the muffler must be removed from the engine to be able to remove it from the machine frame, and possible the pump drive V belt pulley.
