Well yeah, those Jr Dragsters are completely non stock, and of course those GX390s can crank out ~50 HP, but if the GX200s can only crank out ~35 HP, then the flathead wins again in terms of HP per cc.
A flathead made entirely out of aftermarket components, from the block to the bolts.
It's a flathead, loosely based off the briggs design, but that's all they have in common.
That's Like calling a 10,000hp top fuel dragster engine a Chrysler hemi. It's based off the Chrysler hemi, yes, but it's not built out of one.
You could probably get 50hp out of a ~200cc GX style engine as well, if it was built from absolute scratch and designed from the beginning to produce 50hp. 250hp per liter is well within the reach of modern engine design. In fact, when purpose built from the ground up, it should quite easily be able to outperform the Briggs based flathead race engines. It's simply a better design.
The automotive world all but abandoned the flathead engine 70 years ago because it just did not have the same potential as an OHV engine. I'm not saying it doesn't still have its place, but it is an outdated design.