i'm not real sure what i'm doing here in the world of Suzuki guys but this snippet of info may be of some use.
AT1 125 Yamaha pistons are,
56mm std bore,up to 2mm o/s,
16mm gudgeon,
30mm from centre of pin to timing edge of piston,domed crown,
with the AT1 being a piston port motor {no reeds}, the piston would be a full skirt jobbie. would it be possible to trim the skirt {if nessecary} to suit the RM125a?
just a thought.