the 11.5 will probably still be a SPZ section, just the pulley will be worn. There's a wee bit of variation on manufacturers tolerances anyway, as long as the new belt doesn't run down on the bottom of the vee, it'll be alright.
They use a drive belt as they have manual change pulleys for their three speeds.
Clarke, SIP and many others use this generic saw and people like Clarke use what they call an "elephant belt" in a couple of standard sizes on a variety of equipment other than saws.