Sig Wonder
I have the kit for another Wonder in the pile in the spare bedroom. My first Wonder had a 25 on the front and hauled. I flew it for years and years until it eventually died of old age.
My next one will get a YS-45 with pipe and some beefing up to make sure it doesn't come back to earth as confetti.
I would recommend going with a 15 or 25 to start with. Once you get used to that, then consider strapping something bigger/hotter on the front. It's simple to run the front end through a bandsaw and move the firewall back far enough to accomodate a bigger engine mount.
As Gary pointed out, these things can become a dot in the sky before you know what happened.
Regards,
Garrett