I am curious about this as well, not to push you somewhere else but KartPulse also has a forum and it seems to be more active if you wanted to post the question there as well if you haven’t already. I have neither and was considering both and spoke to Swedetech a little on the topic. What I gathered is that the TM is a very solid, fast engine, especially if worked by a builder. They are very reliable and parts are easy to get. The Modena engines are apparently very fast out of the box but I have heard from a couple of different sources that they aren’t as reliable and have had some failures. From what or how significant I don’t know. If I recall correctly, there were quite a few Modenas at Indy in masters and the top 2 were on them.
Your assessment is consistent with what I have heard. I was at Indy running a Honda in shifter masters and my buddy Mark Nagy had one of the fastest Modena’s out there. (I think I was 3rd fastest Honda) Anyway, Mark stuck because the axle moved over and the water pump belt came off with two laps to go in the final running for the lead, so that was not strictly speaking a weakness failure. I decided to order a Modena from Brian Fisher and have him build the motor, as I have been very pleased with his work on my Hondas. I’ll check out KartPulse.