Well certainly lots to think about when considering motor assist eh? At this time outside of the previously mentioned Optibike there are very few bike specifically engineered as ebikes. The rest are just kits that are set up on bikes that are not designed to be motorized. Yes you can do it but as time goes on there will be more bikes that are ground up designs that will not only address the assist but also the stresses the frame, wheels and componentry undergoes along with the extra weight that they bring to the table. As you are already towards the limit of what most manufacturers plan on this can be an issue. Or not, caveat empor.
After having done just about everything you can to motorize a bike in the last 10 years and having many miles under my cleats using one to travel short and long trips I have come to the conclusion that there is no perfect system for all. But one thing I do know is that the 20 mph limit is not enough for a true cyclist to get with the ebike program, especially if they see getting to work 24 miles away, that 25 mph would be better. I like e for around town use as it is unobtrusive and the 20 mph limit is not restrictive in a mainly urban environment.
However I do like to clip along on the more open roads and after having ridden faster, well you just want to ride about that fast all the time. I pedal 95% of the time via gearing on the human side that allows for me to be able to. This requires a pretty high gear range but it is entirely possible.
One way to make smaller engines work more in your favor is to run it through the bikes drivetrain. This allows for a range of gearing that makes the best use of a 1 hp or so class of bike
much more useful in the real world. Unfortunately the regs plainly state that there be no manual shifting of gearing while under power which is why all the scooters these days are automatic also.
So where I am at now is ditching the motorized bike concept and moving up to the moped class which allows for a higher legal top speed of 30 mph. Yes there is a bit more red tape by involving the DOT but it is doable. At this time we are only using 49cc 4 cycle gas motors but it doesn't take a big amount of vision to see this type of bike with a battery setup in the frame cradle.
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