Your problem is similar to anybody driving a car designed in 1995. It aint performing like a 2014 model is it?
While the EVG was a very nice bike in it's day, and the frame from it still is, the motor is a real dinosaur now.
Upgrade to a brushless hubmotor, and you will see a big improvement. Unfortunately though, to get what you really want will take a 48v battery.
There are hubmotors that will go 30 mph on 36v, but you are used to a very torquey motor. If you choose a very fast brushless hubmotor, I predict that you will be very disappointed with how it feels in the first 50 feet from a stop sign.
FWIW, on 48v, a very mundane run of the mill brushless hubmotor blows cheap gas bikes away off the line. But, if you spend a hell of a lot more than $400, you can make a very hot gas bike capable of beating even a hot rodded 40 mph hubmotor.
Before you make any final decisions, you should read up on threads that discuss the 40 mph club. Though it has it's limits, you can make a very cheap hubmotor go 40 mph pretty easy. It just takes investing in 72v. It's cheap and easy to put together a fast ebike. But a very fast gas kit takes lots of money, and lots of expertise to keep the sprockets and chains working at that power level.
Looking closely at that kit, it does look better than the typical 2 stroke china girl kits that are so easily beat by a 48v electric hubmotor. 35 mph from it is plausible. I have seen that kind of speed, and more, from the more expensive Subaru engine gassers. That kit looks like a cheap copy of the Subaru motor.
Nothing wrong with doing both. I have about 6 ebikes, but wouldn't object to a gasser in the garage. You will need another bike to use that kit, it won't easily fit the EVG frame. So why not have one of each? If it disappoints you, sell it and get some of the money back. Along the way, the experiment will be fun.