Recently there was a dude who smuggled a passel of honda exhausts into the country from Not America.
They look really nice and also look like they would more or less bolt right onto your express.
I built a ripper express last spring, but I am guessing you already read some of my posts judging from the fact that you said something about a 16 mikuni.
From my experience, this is the best order for express fasteness:
1. Carb, pipe and reed replacements. The express carbs often don't have replaceable jets, so you kind of need a better carb before you can start with the upgrades. Since you are going to need to upjet and maybe modify your intake some you might as well drop some 2 layer reeds in while you are at it. Also throw a hi flow filter on there and do all your jetting at once.
2. After riding on this set up for a while, get a kit. I recommend either the DR or the malossi. I have used both and liked both. I probably liked the DR better, but its not like way better or anything. I personally want to try the 60cc treats cheapo kit, but thats just me. Go ahead and make yourself a hi compression head while your at it. You are going to have to read up on how to do that because I don;t want to type it all out. Jet in and rip. Also either remove your oil injection or add premix to the gas tank in addition to the oil injector.
After that you are pretty much at the fastest you can make an express without a full engine swap. You can get a little taller tire in the back to get a little taller gearing, but thats really not much of anything. Should be good for just under 50mph. 47 is the fastest I ever dared to go on mine and if I really wanted to wind it out I probably could have squeezed a couple more mph.