The engines are designed for airflow that is perpendicular to the direction of travel for most minibikes. They also don't have the fin density of engines designed to be cooled with ambient air. The shrouds generally direct a lot of forced cooling air around the exhaust port for obvious reasons too. Slanted engines like Hondas have really bad ambient airflow with the cylinder mostly behind the crankcase.
Thank you for your concern. While I agree, in part, I'm certain it'll work out fine for street cruising and the occasional drag.
I remember Freddie Spencer's bike burst in to flames when spilled fuel ran to the dead-air zone of the DOHC head, and my little SOHC 2v builds go 150-200hp/liter and live. A small scoop to help guide airflow to the exhaust port, should be plenty. Fin area per hp isn't great, but good oil and a TBC on the piston crown do wonders, in our testing. We had one kid that would melt the dip stick on hot race days, but got it sorted. Rear exhaust port on Yamaha Virago family lived for 2 decades in production, and them Hogley Ferguson things are a century in to the terrible idea of a rearward exhaust outlet.
I plan to make a billet head any way, and will add generous North-South finning. Exhaust port is getting some welding to fix the bowl (intake too), and there's room to slide a stainless header stub quite a ways in, reducing the heat transfer to the block.
And, there's always the option of turning it around and using a gearbox in place of jackshaft to get it turned the right way. Or sideways with a RAGB. I score a ton of them at recycling. These rascals aren't the space shuttle.
