A quick update for my project..
I ended up doing the rear swing arm modification myself and installing the 900watt motor. I wanted to have someone weld the arm for me, since I have no welding experience but finding someone during the holiday season proved to be difficult.
I became impatient and after looking at the bike and then the motor for awhile, I decided to have a go! I cut the frame with a Dremel tool and then used some aluminum angle stock from Home Depot to serve as a support brace/mounting bracket. Rather than weld it, I drilled through the swing arm and bolted the angle bracket down to it.
It works pretty good! I will have to redo the bracket (not a problem since I purchased a 4ft section of it for $11) because the motor is not 100% parallel with the rear wheel axle. But that's a cosmetic fix as the bike is fully functional.. at 60v!! I am still using the YK42-4 (older version) with 5 12v 12ah batteries tucked away in there. As well, I am using a 14-tooth sprocket on the motor for more top speed.
After four runs over the weekend (all until the batteries seemed to loose juice), there are no heat issues on either the controller or the motor. The original 36v 500w motor that came with the bike would get very warm after half the run time at 60v. I ended up taking that motor apart and there were definite signs of carbon burning on the armature. **Note for anyone else out there crazy enough to try this: as andyS suggested, the 36v 500w motor will not hold up to 60v operation for very long.** At least not with my 200 lb. can on the bike
As for top speed, I'm not sure but it's pretty fast. I hear the wind whoosh past me as I ride now, where before I heard the motor and noisy chain tensioner over most anything else. I think it does over 30 mph, but I would like to confirm this before saying so. I had to remove the chain tensioner to use the larger 14-tooth sprocket, so I don't hear that any more. I LOVE the bike! It still has plenty of torque as well, the next thing I'm going to do is try to adjust the motor timing as Aproldan suggested. Actually, I did adjust it before I installed the motor in the bike with the motor clamped down on a bench vise while running a single 12v battery. There is indeed a large contrast in speed and torque when turned in either direction. For starters, I left it in the original configuration to see what the motor "felt" like.
Overall, this project took me about 6 hours to complete. Not too bad for a bike that no longer feels like a toy. As well, once confirmed I would like entry into the 30+ mph club please!