Hi, If it is the same as my 349's and 242 which I think it is, the spacer you are missing is actually an optional part to replace the original speedo drive when it is removed. Often bikes have the centre of the speedo drive installed which is a cylindrical spacer with a brass collar half way along its length.
The easiest solution is to get one machined up out of stainless steel. It is a pretty straight forward job just dont forget the small ledge on the face that sits against and engages the inner race of the bearing.
You could try South West Montesa or Rocky Mountain Montesa in the US, both are on the WEB.