A good bike engine to use is the GTR1000 Kawasaki engine. Never a huge seller, so cheap. Basically a shaft drive GPZ1000 tuned for higher torque so parts aren't an issue.
The other oddball bike option is the K series BMW engine. Has to lay on it's side, but parts are easy and will go forever.
Car engines... could be too heavy. Depends on the size of the trike and the layout. (sorry haven't seen it)... If a front wheel drive engine could be used, would recommend the suzuki swift engine. Small, light and very pokey. Very popular mini engine conversion. Other than that it's oldler small capacity rear wheel drive engines. These can be heavy though.
