I don't think even the herra chinee radio companies make cassette decks anymore...you're buying used.
I don't remember if your dash is cut for a single DIN unit or not...is it? That'll make your quest a helluva lot easier (and it probably will be a quest to find a functioning cassette deck of any flavor that also fits your discerning standards for correctness) since shaft radios went out of style many years before single DIN units did. Either way, you're pretty much buying from ebay, unless you get lucky on CL or something.
Also, take a look at https://www.diymobileaudio.com/forum/
. I haven't gone on in a long time, but at one point they had a very active classified section, and a lot of really in-depth knowledge on all facets of mobile electronics.
As for cassettes and vintage suspensions, I'd argue that digital media (of the non-spinny-platter type) is vastly superior to any analog format that's viable in a car. Not susceptible to warble from tape degradation (stretch) or voltage fluctuations (not that you'd ever have issue with that in a British car...), not susceptible to temperature related damage, no belts to fail, and you can fit waaaaaaaaayyyy more songs on your phone than you can on that Rod Stewart tape you're planning to have get stuck in the deck when the eject mechanism fails. But it's not period correct, or remotely close to as cool, which certainly count for a lot.
Given that this isn't a daily, consider a small amp, something like https://www.sonicelectronix.com/item_34 ... 300X4.html
, and a 3.5mm-to-stereo-RCA Y-cable, like https://www.amazon.com/AmazonBasics-3-5 ... ca+y+cable
. That particular amp is a 4-channel, but there's a switch that allows it to output four channels with only two channels of input. Stick the amp in a little cubbyhole (I put one under the dash of a friend's 945) plug the other end into your phone/old ipod/whatever, and rock out. Leave the existing radio in the dash. If you wanted to get really cute, you could wire in some switching to also listen to the radio (could get complicated, depending on how fancy you want to get).