You can remove the fill plug on the rear end and wrap a rag around an air hose
nozzle and GENTLY put a few pounds of air pressure in there to help prime the pump if for some reason it lost it's prime.
Do this with the engine running,and the brake applied (locked down) to insure you don't run over yourself!!
Do not subscribe to the theory, if a little pressure is good, a lot is better!
I would @ least change the filter.

The appearance of a filter means NOTHING. They aren't that expensive. Change it, take a magic marker & put the date on it....then you positively know.:IH Trusted Hand:

Just a crazy thought, is that replacement engine turning in the right direction?
It should turn counterclockwise looking at it from the front of the tractor.

It has worked in the past as far as the engine rotation goes to power the hydro.

All small engines turn the same direction. There are exceptions to the rule, but most ALL engines rotate the same direction.

Yes, I realize all engines turn in the same direction. If, however,the drive can be mounted to the other end of the crank it would turn the pump backwards. If it worked with that engine before it's not his problem.

That along with the fact that you can clearly see in the pic the engine is mounted the right way.


OP: Get it primed yet?

Pressure!
 

Good morning friends!
Just an update to my post -
This morning I took off the filter to check for obstruction and found none.
I do have a new filter on order...
Placed the original filter back on and started it up.
Following the priming instructions the pump has fixed itself.
Seems to have had some kind of vapor lock as it hadn't run in over 2 years.

Thanks mates for all the suggestions!
:American Flag 1:

Just a FYI you can use a NAPA 1410 or Wix 51410 filter available ay O'Rielly's or your NAPA auto parts store. It's still a good idea to replace the filter and fluid.

Hydrostat seal at input shaft
 

After fixing the problem with the pressure, there is a leak and yes it is coming from the input shaft seal. Does anyone know if the part is known as the CREEPER OIL SEAL INPUT SHAFT 721-3011 (part number)?
I need to order one and am not 100% sure that is what I need.

Thanks all!