With a .496" rim diameter, one would be better to start with a standard chambering, then do the work, opening up the bolt face and maybe some rail work.
I agree with a Mauser Action - a Zastava would be a good place to start - perhaps.
A re-chambered (& re-stocked) 9.3x62 with the necessary bolt work would be the cheapest avenue - a 9.3x62 Zastava would be one way.
Re-stocking and action work, brings out as much of a 'custom' rifle as desired.