What to use for a garden tractor puller??

That's because they're looking for AC sheetmetal....

Dave's note about the rear-engine Gravely being quite a puller, it is, on account that it has very little weight on un-driven wheels.  Again, I'll refer to the Wismer-Luth traction prediction paper.  Harold identifies with clarity the factors which determine tractive effort.  Here's a synopsys:

1)  Weight of the machine:  Heavier machine will pull more... provided

2) Highest possible amount of weight is resting on DRIVEN wheels

3) Rolling resistance is lowest (pronounced 'large diameter tires')

4) Contact radius is maximized (most tire surface in contact with the ground... provided

5) tire 'climb chord' (the height at which the ground surface contacts the tire... i.e.  if it's sunk down to the hubs, basically all of the tractive effort is going towards trying to climb straight up, not forward)

The only reference to 'horsepower' is the amount required to overcome the limitations above, and once slip occurs, how much more power must be added to cause an incremental increase in drawbar pull.

So on a 'puller', there's no part where horsepower matters until you can convert it from a spinning motion, to drawbar tension.  a 120hp engine will be essentially useless on a tractor chassis where 90% of the weight is sitting on the steer tires, and the drive tires are a small diameter.  Likewise, having a 23hp engine trying to pull a belt that doesn't have enough friction surface to survive the power of a 15hp engine is pointless... and putting 27hp twin into a transaxle whose tubes deflect and fracture under the same forces that an 18hp twin, is simply increasing the cost of the hobby.

There are very few garden tractors that actually have transaxles that were designed for an agricultural field tractor.  The Cub Cadet is one of those.  Doesn't make it perfect, but that assures it'll be heavy, and it is... and they're hard to hurt as a result.

I'm good at hurting drivetrains... I've twisted off three axleshafts on my Loader-Mutt.   Yep, it's a hydro... yep, it's got a Honda GX660 under the hood... I have LOTS of ballast on the back, tires are full of pink antifreeze, there's lots of iron hanging inside the wheels, and I'm not the least bit shy about asking the Mutt to do a ghastly deed... so even the Cub's svelte rearend is in peril. This most recent time, when the axle shaft twisted off (right at the base of the splines, btw), it managed to drop a chunk in the spiders, then get enough action on the diff casting to snap the web loose on one side.  Most of the time, puller-guys end up fracturing the differential carrier... so their favorite mod is to use the 7" Chrysler (from a Volare or Dart), as it's an easily doable mod.

I didn't happen to have a 7" Chrysler... but I DID have a 7.25" Ford Limited Slip from a '92ish Mustang.  I also had a 35-spline Dana 60 full-floater that'd been robbed of it's ring, pinion, and diff carrier, but the shafts and outers were good, so Loader Mutt is in process of getting that LSD, along with a pair of axle tubes that'll have ANOTHER layer of steel around the outside (thick wall tubing outside thick wall tubing) and that limited-slip.

And it'll never see a pullin' track... at least, it'd never compete... because it'd never pass ANYONE's rulebook...

RE engines- doesn't matter what kind, if it was built for a garden tractor, it'll work.  The K-single is like any other engine-  it's just a matter of cubic inches, or cubic money.  I've seen a K301 turning 8500rpm and yielding 55hp on a dyno.  I was NOT standing close, and if I was pulling, I wouldn't bother competing against someone willing to spend that much.  For me, it's not about the machine, its about the operator.  I'd rather watch a dozen 10-year-old kid hook up and try with a bone-stock anything (especially if it's several kids, all pulling with the same tractor) than watch 30-to-50-year-old-men with nitro-gulping forged-assembly tens of thousands on tap roll something spotless out of an enclosed trailer.

My only preference, is that if a kid is gonna go out and pull, that he not be impeded by a weakness of the machine.  He or she shouldn't be impeded by fact that the machine is slipping a belt, or to light to pull, or just plain old set-up-wrong.