Hi Edin,
If your pistons are the split skirt type for the Dragonfly, they will fit a Mark engine but most owners have found them to be inferior to the original Mark pistons. When fitted to the Dragonfly and worked hard, the crowns have been known to start collapsing, resulting in the ring grooves taking on a wavy form and trapping the rings - with the result that the oil consumption becomes excessive. A lot of Dragonfly owners have reverted to the earlier Mark pistons to effect a cure.
On postwar Douglas engines, the quoted bore size is the actual size of the bore - the clearance is allowed for on the piston - so standard size is 60.8mm (2.394") - so +.030" would be 61.55mm (2.424").
Hope this is of some help,
Regards,
Eddie.
P.S. Have a good Christmas - but don't enjoy it too much!!!!