The 98 V70 uses a volvo scan tool like this
https://www.youtube.com/watch?v=DZEoZDjvNMM
and requires a 4 digit pin to enable the car to accept new remotes.
in theory if you know your pin you can program it yourself with a generic obd2 usb type interface e.g. an elm327 ebay job.
you can work out the frequency of the fob based on the part number on the back of the case, however if the case has broken and been replaced the new part number will most likely be different.
the antenna for the fob should be a box attached to the back of the instrument cluster, it will have a big colourful number on it to tell you the frequency it is expecting, but that doesn't help much when choosing a fob.
I think its mostly region specific so as long as its from here and you don't buy one imported from america or japan or something you should be ok.