I wouldn't ever run duals on a 1600, there's no point. Plus Delarto carbs aren't even manufactured anymore. Thank God I still got mine! I'd stick with a single carb on a stock dual-port 1600, which is a very dependable motor. But yeah, I was kinda thinking along the same lines...maybe even an 1835cc motor; if jetted right, dual carbs will compliment it nicely. I run a stroked 1968cc motor with dual delarto carbs, ratio rockers, WD130cam, and some other goodies, nothing special...but it's a blast!
those mini-rails look like fun, I'd say look for a 1000cc motor though if you plan on duning it...little extra grunt on the sand never hurts & if set up right shouldn't be anymore stress than a 600cc motor.