Get yourself a sick bike parts front mount.
http://sickbikeparts.com/front-engine-mount-small-universal/
The 360 grip of the front mount might help keep the engine in place despite the seat tube cut.
Why did you cut the seat post out anyway?
To me it looks like the air filter didn't fit.
For future reference you could have just used an offset intake.
Does your engine mount pass the Shove Test?
Put one hand on your engine head, and your other hand on the top bar above it, and then shove your engine back and forth AS HARD AS YOU CAN!
If it moves AT ALL, that is a fail.
If you can make the engine mount move with just you arms, just think about how much it will move under engine power.