Possibly the european ultralight (don't they call them microlights?) rules.
1. Worldwide, gyro pilots haven't been able to fly on only 35 hp. Actually, Pollini's stats come to 36.5 hp). Once up in the air, yeah, & even a bit less that that in level flight, but not enough horses for taking off & climbing out w/ rotorblade drag.
It doesn't look like there's enough keel clearance to go any bigger. Unless they're using a custom gearbox, the 2.8 ratio on the Polini would work out to about 2850 prop RPM at engine redline. That may be why the best-climb speed is so high.Large slow turning prop surely helps.
I looked at their web site and only the fuel capacity disqualifies it from part 103 operation.The fact that an 82 mph best climb speed is listed in their specs immediately takes it out of consideration for US Part 103. That spec also suggests a long runway and high takeoff speeds might be needed, and the wheels don't look robust enough for a takeoff from "any piece of grass" as advertised.
Yes - The website has been changed since I saw the 82 MPH spec. Jazzenjohn, you're probably right - a mistake in units when first posted.I looked at their web site and only the fuel capacity disqualifies it from part 103 operation. Maximum speed was listed at ninety kilometers per hour (49kts).