Thank you Richard for the thought!
I have my seat matters sorted out. I totally forgot about this thread, good time for an update!
I have a perfect original KZ1300 seat now, including the cover. It was just a tiny bit twisted because someone probably stepped on it, but I got it back in shape in literally a minute. I got it locally in Canada, and was very very lucky at it. Converting Canadian dollars to pounds, it cost me £40.
At one point I will make a list of all the missing items I had to source over the past 9 months. It's a long list. If you can be patient, take your time and once in a while you can find great deals. For example, I got the tail light lens for about 55 pounds, which is about 75 US dollars, and we all know they can go for over 300. I also jumped on another one for about 50 bucks, which I discovered by chance, so I have a spare now. For that price it would be just stupid not to get it, and I am safe now in case one breaks.
As to the seat base, Helmut from MCG in Germany also makes replicas made from fiberglass.
Overall, many of the parts for these bikes are very expensive. If not for patience and good luck, I wouldn't be able to complete this bike paying the regular prices. It still broke my bank, but that's what credit cards are for. I'm just emotionally detaching myself from it after the fact, haha
Cheers!