Replacing the Scout's engine

My scout has developed a big old knock linked to engine speed. Suspecting something in the cam shaft or crank.

So, I'm wondering what to do if it's a major job to locate/repair the problem and am wondering whether an engine change might be cheaper and easier.

Obviously the engines the 8v Tdi 140 PD. Will a 16v 140 unit fit the gearbox/exhaust system, or even better, will a 170 vRS engine go in?

I expect that the 16v engine will be a lot easier to come by as they're fittet to far more cars but I'm wondering if the scout's gearbox will fit (slightly longer gearing than standard cars) and if not how short will the gearing be?

thanks

for any advice

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