don't want to be a buzz kill, but i've done a lot of experimenting with intake manifolds on the HS50. various configurations. make my own intake manifold, but mostly for the look and convenience. i always use a slide carb (16mm to 22mm, just depends)... On the intake manifold, from stock to straight (custom), really hasn't changed anything in performance.
for example just re-did the intakes on a couple speedway 10" wheel bikes. kept the same (slide) carb, just changed the intake from stock to straight 7/8" stainless (with custom manifold plates for the HS50 port and slide carb.) These are stock HS50 engines with open exhaust, slide carb, stock cam, no governor (sometimes a billet rod). Was there a performance difference? absolutely none that i could tell. but it sure looks a lot cooler! It also makes doing a custom straight pipe exhaust a lot easier with a straight intake manifold (another good reason i make custom intakes.)