A lot of the motors these days come with a LARGE mouthed engine mount- I would start there- I don't think there's a lot of difference in the China Girls beyond that-
Maybe you should inquire the vendor about the motor mounts they are selling. Boygofast on ebay has the larger mounts-
I just got a chrome motor from him that has the new mount- It should make installation a breeze- I used to use longer bolts and bend them slightly around the tubes-
On some longer frames, a u-bolt may actually work better, because it keeps the motor at a better angle- you want the carburetor level- I use a straight, short billet intake, so the motor needs to be upright. On the Cranbrook (a Huffy- not Schwinn) probably not, but something like a Kalana Cruiser
I'm not sure if anyone has a spacer adapter to go the other way for older skinny tubes, but we need one!