the flat top pistons will be pushing your compression to the point where it will be harder to run to pump gas, it could be possible but not as easy. the flat tops will have you at around 10.4-.5:1 but the -12cc dished pistons will have you at around 9.5-.6:1. but all of that is based on them being 64cc and not milled down to 58cc. if they have really already been milled down then you will need to go with a -18cc dished piston, which then your compression would be the same as the 12cc with a 64cc head.
here are the 12cc pistons i would recomend.
but if the chambers are milled down to 58cc then you will want these 18cc
with these pistons you must follow the directions that comes with them pertaining to the piston rings and their specific gaps, that is very important.