Hi Simon
I presume you are referring to the studs which bolt the head and barrels to the case, not the nuts ? I can't help directly, but I have a suggestion. If you have a set of studs from a later VW engine, just cut off one end, to the right length, and use a 10mm die (presume you have a tap and die set ?) to cut a thread on the cut end. It's quite hard and careful work, but it will do the job, I've done it myself. If you've no studs at all, either buy a scrap 34hp engine, or buy a set of new studs (for example, see Ebay item 181318634456). If you don't feel confident doing this yourself by hand, any decent professional machinist should be able to do so very easily.
Good luck !