I started with a new Magnum 52 4 stroke on my 4*. Built a custom cowl, but could never get a good idle. It may have been my inability to tune combined with a fresh engine. I sold that and installed a ST51 sideways, ran great. Mounted the ST51 inverted, ran great, idled for minutes with instant transition. Tired of the ST muffler drama and installed an inverted TT46. Couldn't get a reliable idle, mounted it sideways with a pitts muffler. Ran much better untill it ate the carb barrel spring. Tried a fresh Saito 45 inverted, but couldn't get a reliable idle. Mounted the Saito sideways and now have a few hours on it and it is getting real sweet. I may invert it at some time again just to experiment.
I had a Saito 65 inverted on a short lived Venus. It was the only engine I could get to run inverted with the tank mounted up high. Tried a TT46 and Webra61 as well.
I read on here about so many people who never have problems running inverted. I don't seem to have the Midas touch, but I haven't given up yet.

Good luck, MikeB