Richard,
At one point during the mid 70s the Grand Touring model used a SR "AX5" crankset. This had an integrated chainguard - really just a simple alloy ring - which was bolted on way out at the edge of the outer chainring.
I think this was probably the one with the 118 BCD inner ring Sheldon mentioned, - which would have been good for down to maybe a 36 to 38 tooth inner ring. You might still find some of the inner rings kicking around your LBS... but, as Sheldon suggested, you might as well just toss the whole thing it if the outer ring is shot.
Personally, I'd recommend TA 5-pin or Stronglight 49-D cranks. Both of these will take a multitude of TA 'Cyclotouriste' chainrings - from down to 26t up to 64t. AND, they can be set up as either double or triple cranksets, by simply changing the spndle and adding or removing a third (inner) chainring. So, your expensively restored bike can be easily modified if you ever want to just try out different gearing... Or, as you and the bike both age together, the old legs just begin to crave smaller chainrings.
Cheers!
Bob Hanson, Albuquerque, New Mexico, USA