Who built the frame? If its huffy then just toss it in the trash, seriously, aluminum frame by them will not hold up in the long run.
The huffy fortress, you tried fitting in and it was snug, my question is what head are you using, I would highly recommend that frame because it's just a good one. (they have their gems) as to why I'm interested in the head type, well if it's a slant head, you can turn it around and face the spark plug towards the front.
If it's not a slant head, then it's probably a regular head with the plug going straight out, that's easily fixed by purchasing a slant head for under 10 bucks, shipped.
Aluminum frames just don't cut it, unless you can find a good brand, and a bike meant for trails and jumps downhill.
The other guy has a point, the spokes should be upgraded unless you get a clamp on hub adapter.
Steel frames that fit motors can be found rather easily on things like Craigslist, usually you can get an entire bike in working or nearly working condition too.
It's a lot to absorb, I know, but then again, you're both trying to have fun, which requires the longevity of the motor and the actual bike, and the bike fitting your size body and riding style, and also is good for the fun if the bike doesn't disintegrate under you at high speed.
I have a friend who swapped his motor between 3 bikes recently, one frame broke, the next was too big, and the last is OK but still he wants a beach cruiser, so swap 4 will happen again soon.
Most 26 inch frames for a medium to large person will hold a motor. Most stores sell 26 inch bike parts, some sell exclusively 26 inch, because it's so popular.
I would look around just a while longer, I know it's very hard to resist the need to shove that motor into something and zip off into the sunset, but Rome wasn't built in one day. Take your time to do it all right, and you'll find yourself much happier in the end.
(that huffy fortress man, I'm telling you, it fits, and it works, and very robust, and it's a very comfortable bike and size, next bike I build will be that bike beyond a doubt.)