Blow out the air filter and replace the fuel filter. With a paint pen, write the date and hours on the fuel filter (like it is an aircraft). Then go find out if power returns.
If the power is still not coming back, then compression test to confirm the engine isn't getting tired. If compression is good, then I suspect it is time to clean the injectors.
Reasoning: if the HST or pump was screwing up, under a load, you wouldn't lose power, you'd lose forward movement while the engine just hummed along.