Learned earlier today, after spending ALL night swapping engines, that the size 'H' Kreidler cylinder will accept a Garelli stock piston as a replacement-- compression is not as good as the Kreidler original since it is not domed like the Kreider's piston top-- but it works pretty well! Piston dimensions are almost identical.
I will note also, that the size 'D' Kreidler cylinder was too small to house the Garelli piston, sadly.
I've added this and some other info to the Kreidler wikis-- pretty sure I linked everything properly too. :)
Just an FYI! I've been using it all day on my trashed Kreidler cylinder with no issues except a slight loss of 'peppiness'.