G,
That engine from Aero Adventure looks very much like a modified APU to me. Not withstanding the relatively high power output, I'd lay money that the core originates from such a unit.
The Atkins rotary is a Mazda RX wankel, converted for aircraft use. Probably the best expert on these engines is the chap that designed and built my BMW EFI unit for me, Tracy Crook (see
http://www.rotaryaviation.com/).
There's no obvious reason as to why a 12A or 13B Mazda can't be mounted vertically, the rotors are oiled via oil in the fuel and the rotor shaft and gear lubrication should be able to be modified. Tracy has flown an awful lot of hours on his RV using one of these Mazda engines, and is the sort of helpful chap who would advise on other applications, I'm sure.
VP