Why not go all the way and have a flat four. Im sure VW could find some old ones, with legal carbs.
From the news around the next generation Boxster your wish has already come true.
http://www.autocar.co.uk/car-news/scoop/2015-porsche-boxster-facelift-new-spy-picturesWould not fit in the Mog chassis but would be a laugh on the front of a three wheeler