If I had a 125 hp Raby engine I would just take care of it and drive. Check the valve lash once every season or two ($6 feeler gauge, big screwdriver, 30mm socket). Change the oil. Check the timing, keep the carbs adjusted right and change the air filters every now and then. I believe Mr. Pipperato can attest to the motor's durability.
It's a lot less money, time and trouble to learn how to do these few basic maintenance chores than to try to find a "professional" to do them for you at $125 per hour. And sure a lot less than getting a new engine retrofit along with a cooling system, electronic fuel injection & etc.
Having done a Suby swap I will say I am not at all sorry i did, and if I stumble across a Speedie with engine needs I would not hesitate to do another. $5,000 is a reasonable DiY price and, yeah, $12-$15k seems about right with the labor and warranty of a real pro.
My car uses the stock ECU and so I troubleshoot with a $50 hand-held OBDII scanner. I get a code (currently idle air control valve) and I know what component is dodgy. Easy!
That said, I could not imagine that more shops would want to work on a Subified Speedster than an air-cooled one. These are highly customized cars that don't fit into any of the service holes most service holes employ.
P.S.: I was loving that Skunk River site and thinking the shop was totally righteous until I checked the German page. A GEX engine? Really guys? You fix Rolls Royces and Ferraris and you install GEX in customers' VWs?