The ones that are been done in the UK are a exact copy of the original Reed Valve cylinder, the boys had the barrel laser scanned to get the exact dimensions and measurements. You should be able to use all pistons in the cylinder, but I would go back to Mahle because of the coatings and metallurgy of the pistons (expansion and tolerances considerations).
The problem I have is the cost of the cylinder, piston, reeds, cage, pipe and ancillary requirements add to a hefty amount and does the difference justify the cost? Does this madification add another 2.5-3 K to the value of the bike? eg... if a really good Maico 490 piston port in Australia is worth 12-16K is the same bike worth 15-19K with a reed valve cylinder? On the other hand the conversion maybe the best long term alternative to re-sleeving a existing cylinder on a bike that the owner has no immediate intentions of selling.