Having run a 19T a couple of time recently in buggies I've found that they don't pull a big gear like they can in a tourer, the extra weight and the need for more rip off the jumps seems to defeat them.
I would gear the 19 1 or 2 teeth higher than your 12. For comparison I have been running 19s at around 9.5:1 overall in a buggy. This is indoors by the way.
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