Yes you can spin the hydro with a drill motor but it takes at least a 1/2" preferably a 3/4" and not a slow one.
Yes you can carve out a gasket from cork, I've used neoprene successfully also.
But why not used what you bought? first one lasted prolly 50 years.

On your sons pinewood car, be sure to deburr and polish the axels (nails). Chuck in a drill and use emery paper. Smooth any bumps on the wheel outside diameter. Roll on a hardwood floor to assure it rolls straight, adjust as necessary. If it does not roll straight, the wheels will contact the track raised section under the car slowing the car. Nothing hanging under the car to contact the track. Car weight right at the maximum. Lubricate wheels with graphite and spin in. I took a drill and bit to adjust weight at weigh-in. Scouts is good fun for the family.