Just a personal opinion from someone who has worked around a good deal of stationary industrial engines used for generators, water, and hydraulic pump power sources.
You may be well ahead of the game to just toss the electronic system on eBay and get what you can get for it and then use the money to buy a standard issue belt drive mechanical governor and make the right brackets and linkages to attach it to the engine.
A good used universal governor only costs around $25 - $50 if you know where to look.
Maybe less if you dont mind doing a bit of junk yarding at the nearest farm equipment salvage business. Old combines and swather's and many industrial machines are full of industrial engines that have the external belt driven governor systems on them.
I picked several off of scrapped industrial machines at the local salvage yard for a $5 each a few years ago, cleaned them up and sold them for $40 each!
Granted the one you may find may be set for a different RPM range and sensitivity than what you may need but they have adjustments on them for setting those requirements.
Its just one option I would pursue myself being I know where to get them cheap!