Yes National are still in business - the US parent company.
Yes most Presto spares are still listed - including the big washer, the safety plug, and the pressure weight.
No WD40 has achieved nothing so far - but I haven't flooded the inside with it yet.
If I break the bakelite handles I'll probably twist the bolt threads and wreck the thing. So I'm planning to get some shorter bolts and replace the handles with stout bits of wood 90 degrees out from the original handle position - one on one side and one on the other - with big repair washers to stop the wood compressing and bending the bolts - and then to apply a LOT of force.

I don't plan to use it - I just want it working again 'cos I like old things that work. I have a 50 year old electric toaster, and a 50 year old Sunbeam electric frying pan, and a 50 year old electric coffee percolator - oh and a 60 year old electric copper kettle but the one I did junk was a ceramic electric kettle with a plastic lid that was designed to have a live element - so if you'd put a metal knitting needle down the spout you could have killed yourself quite easily at UK standard voltage of 240 volts.