about to throw my hands up and 3rd bike in the trash

Carbide bit on a Dremmel tool, like 1/16" and a carbide tiny ball tip bit too.(for the stud removal) get a good easy-out, the cheap-o ones break and that REALLY suc$s. I had a Riverside also,same issue with front mount. I machined the front "U" of the motor(between the mounting studs) to fit the down tube, I feel the wrap/fit around the frame really helps with the torque load on those #6 fasteners, putting it on a large tube adapter, well the studs take all the side loading, You can "re-size" the engine with a die grinder or a dremmel with a sanding drum on it, takes a while, worth the results. Make sure the engine is seated in the frame evenly, don't over tighten the mounting studs. Just take your time. OBTW I did break a seat tube on the riverside, just under the rear engine mount, you might want to get an extra seat tube and drop/tap it down to the bottom bracket to assist this issue,possibly drill a few holes in the seat tube, like 3/8", then insert the extra pipe and rosette weld it in place,just my experience.

Mac

sounds like exhaust is plugging up

Other then already said, look at these photos...
http://s982.photobucket.com/albums/ae309/Ron-Becker/Engine Mounting/

Dude, I have that EXACT bike! Is it the Riverside 7-speed with the whitewall tires and the glossyblack frame? Awesome bike when the motor gets broken in. I cruise 23MPH on the heavy thing with my 240 pound self on it. Rides GREAT!
Here's what I've learned from mine so far....

1- Get rid of whatever that stupid front mount bolt kit thing you have is. This is how I fixed mine. I cut about a 6 inch peice of pipe and drilled two holes through it top and bottom. Then put the pipe in the existing frame mount on the motor and snug it up a little. Now take the motor and set in the frame and when you have it like you want it, then measure the holes to drill in the frame, remove the motor, drill the holes, put the motor back on and bolt up the pipe to the frame. You barely notice it and once done, you can remove the motor just as easy as the V-frame bike it was designed for. So far it has held on tight for a few hundred miles.
2- The fenders will mess you up if not tightened down with backup nuts! Had the rear one just suddenly rip out and luckily it didn't catch the chain. Just started dropping parts on the bike trail! LOL! Right now I removed the front fender because it started making a scary noise and its something to do with the crimp underneath. Haven't looked into that yet.
3- get a dual brake handle for the right side with a long handle and not that little short handle that the bike comes with. This thing needs LOTS of brake because of the weight and the speed. This is my next project to it because the back brake shoes are always needing adjusting from wear because they are the only brake.
4- a dry box from my boat screwed down to the luggage rack works great as storage for my bike lock, tools, oil, etc. Placed just right, I can go get my gas from the store with a 1 gallon tank placed between it and the seat.
5- Seems that the NGK BH6 has made my motor run better. The original LZ4 just didn't sound right at high speed as I leaned out the oil mix. It is hot and humid here maybe that's why cooler plug is helping. Not sure.
That's about it so far. This is my first bike to put a motor on and I have learned a lot so far.

Here are some I made for our bikes.




Here is one that I bought for a Chopper...