I've built probably 3/4 of the 1/12 Tamiya motorcycle range - and they're all pretty similar - extremely good fit and very well detailed. A little extra detail added can make all the difference in the world.
I've also built a couple of the 1/6 scale bikes - they're a real knockout - especially the Suzuki Katana. What a monster. A massive model of a wicked bike (well, it was, for it's time anyway) and it looked spectacular.
I sold it long ago, and I think the gauge was a decal, from memory, but I painted out the white needle with black, made a scratchbuilt needle and placed the clear cover over the top. Looked fantastic.
Really, no matter what you do, a large scale Tamiya kit will always be good value and should build (with care) into a spectacular display piece.
I use several different methods to replicate seat material - whether it's supposed to be vinyl or fabric - using flat paints can replicate fabric, and using semi-gloss Tamiya X-18 is a good paint to use for vinyl. I also modify the way I apply the paint for different finishes - spraying thicker paint at lower pressures can produce a "spatter" effect which can be good for things like vinyl seats or vinyl tops on cars. A thinner paint with higher pressure gives a smoother finish.
Experiment a little to find the best settings for your system.
Good luck.
Cheers
LeeTree