Remember were talking dinosaur big single un-counterbalanced motors here.
Even your 200 Blasty's came with a counterbalencer for good reason. My wee 400 from 79' is the smallest of these beasts.
Next size up (80' 465) is 65cc's more, then they made it (82' 490) 90cc's more. That's alot of piston swinging around in there.
It's the vibrations. A 490 swap will break your welds. It will crack your pipe and frame. It will hurt your hands on a long ride.
Even my 400 vibrates enough to squat the rear shock. If you don't know that means it actually makes the rear shock compress without hitting a bump standing still.
It will cure your problems with constipation though haha
Besides the lack of a counterbalance, it's the stroke.
Lets say you took a bone stock shee and put an 82mm crank in it like the 490 has, You would have a 527cc banshee.
Here's my lil piston. Imagine feeling that thing eveytime it reaches the top and bottom of it's stroke @ 9000rpm's...PB