Most the guy's here (unlike me) live in America I imagine a set of rebuilt magnum heads couldn't be that much ? Where if the OP wanted to could do a magnum head swap but I guess aluminum speedmasters are so cheap it generally don't make sense.
About 700 bucks for rebuilder type stuff off ebay with free shipping.
I doubt a local machine shop would be any cheaper.
I wasn't aware speednaster was doing magnum heads.