I installed a DDM big bore kit on my 23cc converting it to a 27.5. Very pleased. I did note that it wasn't super clear which way to install the piston, and although you did say everything was installed correctly, maybe you overlooked the piston orientation?

Another possibility might be the cyl bolts. My engine only has two, and although the cylinder does slide into the crankcase, I suppose if you torqued one bolt fully before tightening the other one, a little canting might take place and cause an air-leak. Oh, I left the white paper outer covers on the cyl gasket...I think I read you're supposed to do that.

One more thought. You would have had to move the ignition coil from the old cylinder to the new one. The air-gap between the must be set correctly and if the cylinder has paint on it, some must be removed before mounting the coil to get a good ground. Of course, you said you had spark, and as I doubt you removed the flywheel, the timing should remain correct. Maybe double check and make sure the coil mounting bosses are in the same place from the old cylinder to the new....if they arn't, the timing will be off.

Don't take offense, but if you did everything right, it has to start. I've made goofs before and ran around in circles trying to figure out what was the matter because I was so sure I did everything right. In the end, I generally find I had made a mistake.

I'm sure you can figure it out, so maybe try again and this time it will work fine.