At the front just poke the forks up through the yokes by an inch or so. This is a fairly standard trick on enduro bikes for those with short legs. Alternatively you could perhaps use the forks from an 'ordinary' 650 but fit your 21" wheel to them?
At the rear why not buy a standard 'ordinary' spring/damper unit and use that?
I'm not sure how the geometry will finally work out but, if lucky, you may even be able to find somebody with the 'ordinary' 650 who would like to make it taller and swap bits with them. This could then be a no-cost exercise.
Try ringing BMW in Bracknell too. They have some specially lowered 'ordinary' bikes on their offroad course so may be able to give you some advice too.
Andy
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Andy
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