I currently have a 73 Westphalia. Gone is the wheezy, hot, tired VW engine. It has a Toyota hemi (1600 2TC) in it now and it will absolutely fly. It cruises easily at 70-75, flies uphill in fourth gear and will easily pass others on the open road. It averages about 22-23 mpg. The nostalgia factor is great and it always gets attention. The radiator is mounted parallel to the engine in the back, so no one knows what is powering the bus.
This is a true, genuine hybrid since it is an amalgamation of Toyota and VW and because of the engine design it really is a hemi. It is affectionately called the Toyobus. I feel I have the best of both worlds.