On a full suspension bike, I would think a "rack mount" would be your best option although the cost goes up a bit. (Rack mounted is when the engine is mounted above the rear wheel) Many dealers here sell them. Be sure to ask them before you buy if it will fit the particular bike you want to use.
Also in the DIY section, some folks do amazing stuff with weed whacker engines and the like. Deacon has a booklet for sale.
His front wheel drive would work great with a full suspension bike. Be sure to read some posts by "Comfortable shoes" as well.
I should warn you though, these things start out as just a good idea. They then turn in to an obsession, snork
Please let us know how your project goes!