Bill mentioned a constant speed prop.
Here is something I saw at Aero.
Does away with the $1000 Woodward Governor
and drastically simplifies the PSRU nose.
No need for a Gov. gear driven mounting pad on the
nose of the PSRU.

A small electric pump controlled with a 555 PWM
could provide oil pressure to a stock constant speed
hydraulic prop. A high pressure positive displacement
fuel pump like the Walbro or the Airtex roller vane
pump could do the trick.

All you need is a rotating coupling to get the
oil pressure into the prop. There could be a way
of modifying Tracy gear box to do this.

Paul Lamar