If you don't get the upgrade from NEC then you will have to get a second hand processor! Nec wil not cover that as the software is sold on a right to use license! Basically you are screwed if you want to stay legal! However I have to ask just why you need to replace the processor if everything else is working OK? If the RDA facility is down due to a damaged processor, so what!
You have to evaluate what financial burden a processor that won't allow remote connections will cost you. Then if you aren't using SMDR off the other processor port, you have to consider if it would be cheaper to provide an external modem service.
If all considered options don't justify it, just accept that on occasion you may have to change the connections between the CPU and the Modem!