Hi Alan,
Have you checked the end float of your crank assembly? I'd be surprised if a gasket is required between the crank case halves, but it may have been put there due to lack of end float. If there's no end float problem, I'd just use a sealant between the halves. I have good luck with a Loctite non-setting type - it's black goo with a little bristle brush attached to the lid. If you need a bit of clearance, maybe use a dry brown paper of the required thickness, perhaps lathered both sides with said goo.
Leon
