It might be a little harder than you think. Satellites typically use some kind of encryption on the command channel, since no one wants to find their satellite has been hijacked. Even back in 1978 that was likely the case. So when they say 'decommissioned the transmitters' what they probably mean is 'threw out the data encryption hardware.' If they also threw out the design files (and remember these are the people who shredded the Saturn 5 engineering plans), there's pretty much no chance of talking to ISEE-3. No matter how many enthusiastic and capable hams try. Radio transmitters and antennas are easy. Yes, hams could do that part. But the packet structure and encryption - not easy.

It's not a case of hacking a protocol, or breaking the encryption on a given stream of data. There's no data stream. Unless the encryption is correct in commands sent to it, there'll be no response at all.

There might not be any encryption on the return data channel. Nothing anyone can do with that which harms the mission, so why bother adding more circuitry to the satellite?

Give NASA another brown star.