is it at all possible the extra displacement comes from the crank? the crankshafts in each of the cases are definately different.
THAT IS THE CASE INDEED

Yah , then you need the 026 crank indeed
But still ( PUT THE PISTON AND CYLINDER WITHOUT THE RINGS LOSE TOGETHER AND SEE IF IT TURNS ALL THE WAY )
and if it turns all the way, then put two bolts to hold it in place and put a piece of solder rod in the spark-plug hole,
turn the engine (with the hand or tool) through the compression and measure it, then you now what feet-gasket you need
Cheers Saul