With my professional hat on (I'm a sales engineer for WABCO we make discs for trucks), i'd really really encourage you not to do this.
The cast structure and surface of the machineing is critical to the performance. Stainless steel is in any case second only to chromed steel for it's rubbish performance. Stainless is used for vanity with oversized pads to make up for the poor structure and lack of surface retention (it wears smooth, while the holes in cast iron means it wears rough). Get it wrong and you will either fail to stop (hot or cold performance could be off) or get hit with shrapnel at some critical point (heat spreads cracks), or both.
Please buy either the OEM fit item off e-bay or a cast iron replacement.
If you really must do what I consider something dangerous, at least use cast iron (it'll fail on the lathe if it's going to go bang properly on day 1) and get your machineist to copy the surface finish on an un-rubbed section of the old disk, or make it rougher than a piston bore would be (it'll perform with your new pads unless it explodes). Go for the minimum amount of cutting and avoid all holes not strictly required for mounting. There should be no fashion accessory type "cooling" holes. The grades of cast iron vary with the design of disk, most are speced by each manufacturer to match the casting requirements, they are not commercial grades. Manhole cover type stuff is the right ball park in most cases, rough grey cast iron.
If it looks too home made and the bike was made after about 1998 it might fail it's MOT for the lack of E-mark on the disk, but not many MOT places would spot this.
I really hope I put you off doing this, but also really hope you get sorted.
Andy
