It's been a long time, but as I recall all the T-300 bikes (which include your Speed Triple) used basically off-the-shelf fuel filler caps. I'd take your old one in to a Cycle Gear and compare it to what's on the shelf. You might get lucky. Otherwise, do a little online checking. Personally I wouldn't bother if the old one works. Your bigger problem is going to be the vacuum fuel cut off valve. That was problematic from the beginning, and they tend to freeze up over time. There is a replacement available from another bike but the real solution is to install a manual cut-off/reserve valve. It doesn't look right, but it will always work.
For the record, the factory Mikuni carburetors are flat slides as well, but they're a CV (Constant Velocity) flat side. What you are installing is a manual flat slide. Those Keihin flat slides were a popular upgrade back in the day. They have their advantages, but don't expect a dramatic performance boost. The biggest change is going to be smoother throttle transitions, but the biggest advantage is they maintain tuning better. You absolutely want a push-pull throttle cable, but I don't have a recommendation for you.
I honestly wouldn't let anyone talk you into a 3>1 exhaust either. The performance advantage is minimal, and it actually narrows up the power band. You're already going to be losing some mid range by eliminating the airbox with the new Keihins. Those slip-ons you have appear to be Jardines, which are light and decent performers, but they always look like ass after a season. There was a day when you had a lot of options, but now not so much.
It's a shame to do a lot of mods to an old T-300 Speed Triple. They're getting hard to find.