vertical shaft hondas are used on lots of smaller zero turns (riding mower) and are air cooled (legal) and can be sold as replacement engines for blown up inteks, vannies, kaws, kohlers, and indians for riding mowers... so whether is vertical or horizontal (seen some of those as repower kits in 300 and 400 series john deeres ie riding mowers) non liquid cooled v twins and you find or have a flywheel made for it yes they are legal... but with aftermarket parts availability being key factor whether anyone runs one or not is the biggest issue.