There's more to the gravel kit than just the high float gear. The belly must have a fibreglass protection strip and I am not sure whether the nose gear must have a deflector or special chined tyres to stop debris going up into the engines. In any case, although much of the development work was done when the Poms were trying to drum up interest for the aircraft in developing areas, it never got certified as far as I am aware. Probably because they needed a minimum order of 10 to pay for the approvals and most countries in the market for such a beast could only afford to buy one or two at the most.
Not to say someone couldn't pick it up and run with it, as all the design work and initial performance data has to be out there somewhere. But dealing with the PNG CAA or CASA to get it approved for commercial operations? Good luck!
Pity, because the Avro RJ would make a great semi-STOL dirt runway jet in a specialised application. But, expensive to operate, so it would take a Lihir Gold to afford it.