In the Metafun manual source the setup for page numbering is:
\startuseMPgraphic{circled}
pickup pencircle scaled 1mm ;
drawoptions(withcolor (.6+uniformdeviate.35)*white) ;
fill fullcircle xscaled 1.5cm yscaled 1cm ;
drawoptions(withcolor .625yellow) ;
draw fullcircle xscaled 1.5cm yscaled 1cm ;
\stopuseMPgraphic
\defineoverlay[circled][\useMPgraphic{circled}]
\def\Circled#1%
{\framed
[background=circled,frame=off,offset=overlay]
{\color[darkred]{#1}}}
\setuppagenumbering[location=]
\setupheadertexts[margin][][\hbox to \hsize{\hss\Circled\pagenumber\hss}]
and for chapter titles:
\startuniqueMPgraphic{titled}
path p ; p := unitsquare xscaled \overlaywidth yscaled \overlayheight ;
pickup pencircle scaled 1mm ;
drawoptions(withcolor .625yellow) ;
draw llcorner p -- lrcorner p ;
setbounds currentpicture to p ;
\stopuniqueMPgraphic
\defineoverlay[titled][\uniqueMPgraphic{titled}]
\def\ChapterCommand#1#2%
{\setbox0=\hbox to \leftmarginwidth{#1\hss}%
\hskip-\leftmarginwidth
\hskip-\leftmargindistance
\framed
[background=titled,frame=off,offset=0pt]
{\box0
\hskip\leftmargindistance
#2}}
Hope that helps!
Duncan