Finding replacement rods is a problem that many of us face in keeping our old bikes running. Obviously the best way is to have NOS but as time goes on this seems almost an impossibility. Like others have mentioned alternative rods from other sources i.e. different brands, go carts, jet skis, outboards etc are a possibility but its still very hit and miss and its a lucky dip really.
I guess if access to a well equipped workshop is available things such as different pin diameters or the ability to modify rod widths at either or both ends are a possibilty. I'm not so sure about billet rods though. With CNC machinery this would be an easy way to make a one off from billet and suitably heat treated and ground, but without the grain flow that is inherent in forged rods I'd say the risk of rod failure in a highly stressed engine would be highly probable. Would be good to try it but who would be willing to suffer the catastophic damage of failure.
I have been able reclaim one rod (elsinore 125) that had a small amounts of wear by having the big end bore ground to clean it up and then make a matching stepped crank pin to restore the correct running clearance and using the standard needle bearing. The case hardening in this area is usually deep enough to allow such a mod provided the material removed is no more than 0.2-0.3mm on diameter.
Wish there was an easy solution to this.