As I mentioned the other day, do make sure that the rattle is not being caused by the gudgeon pin holes in the piston/s being worn oval due to the pins being seized or partly seized in the little-end eye of the conrod, which makes the pins turn in the pistons rather than the conrods. (They are supposed to be an interference fit into the pistons). If you do have this problem it is MUCH cheaper to get the piston gudgeon pin holes bushed back to being round, and a good fit onto the pins. Even if only one is worn get them both bushed so that they are the same weight.
In any event, new 498 pistons are not currently available, but may be, (from the Spares Scheme), in a few months time.
It might be possible to find a good pair of used oversize pistons, at perhaps plus 50 or 60 thou”, and then get them turned down to fit your bores. The bores themselves of course, will need checking for wear and ‘tramline’ damage, plus honing to remove any ‘glazing’. All this really needs doing by a Scott specialist repairer, rather than a local engineering company.