I’m very happy with my experience with Dan (Woltjer). I purchased one of his KA100 rentals after the 2019 SuperNats for a very fair price. I went with Dan because after a quick conversation he understood my mechanical ability to do all the assembly work and agreed to do as much or as little from a work standpoint for me as I needed. I found many builders, to protect their reputation (which I respect), don’t want the end user to do anything on the engines but put gas in them and turn the carb needles and then send the whole thing back as a means of service.
I sent the engine back to Dan for a full top end before the 2020 season started and when it needed refreshed again mid season I was happy that I could send the jug to him to have it bored to fit a new piston and he sent me all the parts back with an extremely quick turn-around for me to assemble and break-in myself.
Dan has never failed to jump on the phone or return a call when I have a question and his wife does a great job of running the front office
As TJ stated, there are alot of great builders out there and quite a bit of parity with the newer engine packages, your particular needs and / or your location will be a deciding factor as well as presence at big events near you that you may be attending.
Anytime we talk about builders I think it’s worth pointing out that Christian Fox (Fox + Son Karting) is an active contributing member of this forum and have been a great asset here and can without a doubt build an engine as stout as anyone in the industry.