I've got 2 engines that are gonna end up in garden tractors. I have a 316 (onan) without the engine. Been stockpiling parts (twin brake pedals, 3 spool hydraulics, etc) to turn it into a 318 super-clone.
I had picked up a Briggs 18hp vanguard twin to put into it and had started doing the layout for that install. Read most of the threads about those conversions and it seems pretty straightforward...
But then I picked up a Kubota D600 (16 hp 3 cylinder liquid cooled diesel) in a golf course mower and I am tentatively switching to swap that one into the 316. The torque, water cooling and decreased fuel usage of the diesel win the comparison, but the install will be much more involved..... The donor has a power steering system as well, so that will be going into the 316.
A small liquid cooled diesel is, in my opinion, the "best" engine option, but may be overkill for most garden tractors unless you use it a LOT. With any kind of diesel engine, I suspect traction and (lack of) weight will be the liminting factors, not power..... In my case, I want the 316 diesel as a back up cutter (for the bigger tractor and the fields - we have a fair amount of land....) and portable power source to be able to run a generator or pump or ???.
The 18hp gas twin will most likely get relogated to either my 316 kohler or a ford LGT160 - both have single cylinder kohler shakers in them, and the smoothness, lower fuel consumption and added power (compared to the kohler k341) of the gas twin would be an improvement. The gas twin also weighs less, so adding it to a non-power steering tractor lightens the front end.....
Not sure what the twins fuel consumption is compared to the original onan, but unless you use the tractor a LOT, gas $$$ isnt that big of a deal...
The onan twins were good motors - meant to last and providing good power. If I had a rebuildable one, spending the money to get it up to specs and running good might be the easiest option. Unless you get the repower engines really cheap (which was my situation...), you are going to be spending serious money either way you go...
Thats my two cents worth...