Welcome to the forum! I tend to lurk on here, so don't expect to see tons of great informational posts from me. I've still got WAY too much to learn...lol
I don't think turning a Sportster into a cafe racer is that odd, old or new. Done right they look great! Check your messages for some more information on H-D XL's turned into cafe racers... I didn't want to go posting links to other forums on the open forum here since this site will be able to help you more on true cafes than the other forums I mentioned to you. They just happen to be better at H-D specifics on them. Anyway...
You sound already ahead of the game with finding sources for some parts already. For the seats you can either buy one of the pre-made ones fiberglass pans (it's the easy rout but you won't stand out as much) or build your own (LOTS of information out there on how to do that). For the tank since you don't want the peanut tank anymore, and I don't really blame you, you will either be having one built, buying a pre-made (and often fiberglass) one, OR fitting one from a Japanese bike. If you like I can PM you with a few different ones that I have seen that fit with some mild modification. Mainly Honda and Yamaha tanks. If you don't need/want your old tank I would suggest selling it; I have seen them go for a good price lately. The exhausts that I have seen that look the best in my opinion tend to be custom bent, but again there are a few different companies out there that make what you are looking for. A lot of the cafe Sportsters out there use Suppertrap cans.
Remember, anything that you can't find can be built!
Here are some more H-D's to give you an idea of what you can do. I have a ton more saved on my PC since I am hoping to get an XL883 (cheaper insurance! lol) to cafe. I can always do the 1200 engine upgrade later.
Oh, and I only included NON-peanut tank cafes for your viewing pleasure.
Even though I warned you in my PM, I'm going to do it on here as well for all those thinking to follow a similar route as you. MOST "cafe racer" Harley-Davidsons that you find out there have "flat tracker" seats and bars. Yet many still call them "cafe". To many they are so similar in style that they are the same, but to purists out there they are vastly different. So just make sure that you have a clear idea of what style you want and call it as such.