I sort of put this together for MirageRally.
This is what it looks like riding a bike like the Honda CB500X -
https://www.youtube.com/watch?v=IlhR_BuoOJ8
I really like the 471cc engine in these Honda bikes.
This is the view on a Honda Grom -
https://www.youtube.com/watch?v=BlhZndsy8vE
Note: It can't quite do 50 mph on the highway according to this guy. I would have concerns about putting a 471cc engine in cycle made for 125cc.
This is a ride on Honda Rebel 500 (same 471cc engine)
https://www.youtube.com/watch?v=CbeUWzQvcNI
I prefer the upright sitting position of a standard or adventure sport bike. Thus, I would prefer the CB500X, but the Rebel 500 may be something to consider.
Adding a 500cc engine to a tiny Grom doesn't seem safe to me. That's a lot of power for a very small cycle. I honestly feel you would have more control of that power in a Rebel frame with brakes & tires that are designed for that power, too.
I am not really opposed to a Grom. They look like fun putt putt bikes. In my opinion, they seem like mopeds on steroids. I think it's the modified Grom that concerns me. Just saying as a friend to a friend.