I really have never heard of them being made for DSM's. But:
a: You have a n/a.
b: They are NOT necesary for road use.
c: If you did buy them, you would see little benefit.

i read that the evo8 has them. and the evo runs with a 4g63 engine

This is old tech.

Some LT1's and all LS1's have them.

If you have extra $ for a durability mod I say go for it.

http://www.ganioglu.com/supar/sodyum/sodium_filled.htm

The STI and EVO have them. The engineers did'nt do it because they were bored.

An engine run hard will last longer with them.

80-150 degree valve head temp drop combined with at 7%-16% weight reduction.

If you do it do bronze valve guides at the same time.

They run cool enough to allow tighter guide to stem tolerances.

That company did'nt had mitsu valve but did'nt look like 4g63 valves. They likely have them however.

There should be other manufacturers. Na (sodium) filled valves were on production engines even in the 70's. It's probably an relatively untapped mod for these engines and worth it if you need new valves and guides already.

The scientist/engineer side of me says do it. The home garage tweaker in me says only if I have to work on the head and replace stuff already.

Anybody have CAPS?
IS
MD-127 841
a 4g63 exhaust valve

If so their part
53050
30.5x6.6x109.7

Will work for you.