Looks like you will need to make a special mount of some type or if you don't care about chopping and welding on your bike, weld in some tubing to make you a nice engine mount. I think you have the start of a nice bike. With a welder you can motorize almost any bicycle.
You might also have issues with the first chain & chain guard hitting the side of the engine, unless you mount it up high.
You will probably need a extra long chain too.
Look at onesikschwinn's first black bike... look at how he offset his engine to the left side. That might solve all these problems & work well on this bike for mounting your engine behind the cranks & clearing your tire & chain! (if you pedal clears the side of the engine.)
Check out the ones I just posted to the picture gallery. These are ALOT of work, so you will have to be patient. I was able to build 2 over the winter, but it was very time consuming to overcome all of the clearance issues. Hope these give you some ideas!! Good luck.
I have seen Switchblades that have been done on the Spooky Tooth site, they look great. They had their motors mounted low so that the pedal chain goes around the clutch cover. I think that it would look better mounted high and forward, above the pedal chain. I have a Switchblade just like yours that I use as my pedal bike, out of my stable of 20 or so bikes this is my favorite rider and I think it would make an awsome motorized bike...Kelly