hopefully this helps. i assume you mean the hs-142f 4-stroke kits. i have the hs-142f on a semi-mountain bike. it has a slightly curved top. the downtube isn't straight but slightly angled towards the back wheel though. the engine sits on an imperfect tilt to the rear sprocket. i had to also remove the muffler cover (useless anyways), carb filter box (just need foam over carb intake), and remove the throttle assembly on the carb (made my own).
because the engine sits on an imperfect tilt, the chain needs to be adjusted with additional chain tensioners to compensate for the tilt. the cardboard sizing cutout (no plastics removed) from bikeberry is very accurate to check before buying the bike. having an imperfect tilt is not the end of the world but i would definitely not recommend it for a customer or anyone who just wants little hassle.
but, there are some larger mountain bikes out there that i think should fit with little plastic removal and no engine tilt. but, i mean you have to be taller than average for those bikes.