Sorry Bob.. I have no direct knoledge of the wilesco but mamod pistons/cyclinders always seemed loose to me.
if over tight than it won't run , if its loose it will run ( loosing steam yes but will run) , if it has the kind of seal you seem to have cold and both are brass then should still work hot ( if dissimular metals then a check for relative expansion rates might be in order)..
I remember a "rule" for small engins..if the piston will fall under own weight "unchoked" but oiled and will make "pop" noise when pulled out of choked cylinder then thats ok….
Best of luck