I have made and used a ram air scoop on my '80 Jawa Babetta for the last couple of years. It has the small Jikov carb (9 mm, I think, as opposed to the larger
12 mm one). My 'ped was always sluggish on the top end (20 to 25 mph),
as if it were trying to get over a little hump but couldn't. I made up a 4-sided pyramid kind of thing out of cardboard (taken from a Corona Extra case!), "weather proofed" it with black duct tape, and taped it onto the end of the air box, which on the Jawa faces down and back a little. The air scoop curves around so it faces forward, more or less, and I angled it a little to the side so it isn't hidden behind the front wheel fender. I'd say the opening is about 4 inches by 4 inches. I really think it helps. It gets the 'ped over the little hump so I can cruise near 30 mph on the flat (as long as there is no carbon buildup in the exahust port or crud buildup in the muffler). At low speeds, of course, it does little or nothing.
BTW I also tried something else before that, but I can't be sure whether it helped much. In the top end of the air box (more like a rectangular pipe, really), I drilled and tapped three or four holes for 6-32 thread. If I put quarter inch screws (with washers) in the holes, it's the same as their not being there. If I remove one or more screws, this lets more air into the system. With all four screws removed, I could definitely tell that performance was suffering. With only one or two removed, I got the feeling sometimes that it helped.
However, the air scoop definitely helps. No question. Plug chops seem to reveal that it's not running too hot. Besides, I've put at least 1500 miles on it with the scoop there, and I've had no engine seizes or other bad stuff. Maybe the exhaust port carbons up a little faster, but I can't be sure of this.
Right now my 'ped's not working well. I'm pretty sure it's a clogged fuel filter, which is absolutely essential given the rusty gas tank. I'll check this out by making a temporary fuel tank out of a coffee can + quarter inch dia. copper tube. I'll lead this temporarily to the carb. fuel intake and see if the 'ped runs smoothly at mid and top end that way. If yes, it's a fuel flow problem, and I'll replace the filter (teensy one made for small engines, I suppose. I found it in an auto parts store,but they don't carry it any longer). If not, then maybe the carb needs cleaning. I've got a bigger carb (Italian 12 or 14 mm one) that I'll try sometime.