You'll have to check the 412 spec. and other such requirements to acquire a suitable replacement. As tantalizing as it may seem, the 410 spec. and the 340 spec. are far from suitable for this purpose.
Just kidding.
Isn't the voltage and amperage on a sticker anywhere on the modem?
Viking doesn't make this stuff anymore, and hasn't for a while, but you might try to contact them.