IMO, a CT70 engine is just too large and heavy for a bicycle frame. (~50 lbs) You'd have to remove the pedals too, which would make it a motorcycle.
If you decide to rig your own custom mounts, you will have one fast bicycle!
There are literally millions of these things in garages and workshops everywhere because they've been made since the sixties. Manufacturers in China are building them now too, since any industrial patents have long expired. An original CT70 engine is 72cc and probably has a three speed semi-auto transmission. This style of engine can be had as small as 50cc, and as large as 140cc. But even the 50cc has the same mounting footprint as the larger engines. My Trail Cub clone has a 110cc engine w/4 speed manual transmission.
arceeguy is correct. There are tons and tons of these scattered about. A motorcycle repair shop near here has several in various displacements from 50 to 110 cc. I'm either going with a 50 or a 70. Leaning towards a 70 because of greater parts availability. BTW, I intend to leave the pedals on. And yes, it will be pretty fast. The bike has Atom drum brakes front and rear now. These brakes have been widely used on mopeds and tandems for years.
I have one of these in 110cc version on a youth quad. It really is a nice, smooth, torquey engine. I would think it would be fairly simple to fabricate a mount to clamp on the frame out of some thick plate or angle. May need to weld a bit, though. For $200, it's a lot of motor. I am thinking of buying one to use on a gokart or cyclekart.