If you were to re-engine a Kolb Firestar with a 4-stroke engine, what would you do?

Firestar is a quasi-ultralight that does not quite qualify for the Part 103 limits and is usually available as pre-owned EX-AB or grandfathered E-LSA. Kolb's heavier 2-seaters use the Rotax 912, but I'm generally curious about something below 50 hp.

I just read on his blog that Jim Wiebe tested a Belite with 1/2 WV by Hummel. To get there within the Part 103 weight, he had to do a major rework by switching to an all-aluminum construction, so I do not expect to see many ultralights under 4-stroke power. Still, it is very intriguing... We have a regular fly-in that I often attend in a rental that is about 2.5 hours away if you make 60 mph. I see many benefits from increased range and larger TBO, just wondering what the costs are.