Tascott,
If you're very careful and feel confident using a Dremel tool with a metal cutting burr or a sanding drum, you can alter the inside diameter of the front motor mount so it will rest against your downtube. You won't need to remove too much material, not enought to compromise your engine for it to fit. Instead of buying or making studs you can use bolts of the correct length. Allen head cap screws are available at Ace Hardware and other suppliers that will be better than all-thread which is usually a much softer material. Most Allen cap screws you'll find, especially if they're black, will be a grade 8 bolt. Do not use stainless steel fasteners on any high stress application such as motor mounts. Don't overtighten your fasteners. Tighten evenly from side to side and only until you flatten the lock washers then a little tweak more. Hope this helps you out. If you don't feel comfortable grinding on your engine then there are aftermarket mounts available.
Tom