The Yamaha Virago uses the same wheels as the Knock. Well, almost. The 1996 Virago I have uses a 15" rear and a 18" front. The rear uses a 14mm Axle bolt same as the Knock and I believe the front uses a 12mm same as the knock. Not 100% positive on the front wheel. 100% on the rear. The hub is alot wider on the Virago pushing the sprocket over towards the left a good bit. You would definitely have to make new spacers to get it to line up correctly with the knock motor and you would probably have to have a rear sprocket made as the Virago sprocket is alot bigger.
The Virago wheels are just barely narrower than the knock wheels. Maybe 1/2 a millimeter. Not much.
Most knocks I've touched, the wheels are fine, it's the tires that suck balls. If you get wheels the same as you knock, that won't fix your problem as you will have the same tire choices. If you go with a 16" wheel in the rear, you'll have to change fenders and probably move/replace the frame cross bar supports so the wheel and tire will clear.
Are you sure your wheels suck and not your tires?