Maybe I'm just too deep into this stuff, but that doesn't seem like that hard of a problem to solve. I've been doing digital manipulation like that for almost 30 years. It shouldn't be too difficult to figure out what the speed difference of the tape is. It's likely a simple fraction. I don't know about the speed a compact cassette runs at offhand, but the standard speeds of open reel tapes are 15 ips, 7 1/2 ips and 3 3/4 ips.
And now because my curiosity was piqued I went looking. Standard speed for a cassette is 1 7/8 ips and 15/16 ips is a common speed used for talking books.
And if you really wanted a 4 track cassette deck they are available even today. Digital manipulation is much easier though.