Because that's how the Brits did such things and its a good system. That collar does a lot more than go round and round on a free ride. It is taking the end trust of the cam which can be considerable given Indian machining tolerances.
Roll pin in a pinch? maybe. Cotter key or spring pin? No way.
Do yourself a favor and use the right part, a #6 taper pin. Gary at Diesel Electric should have them in stock, if not at Mcmaster Carr the part number is 98390A343 for 1 1/2" long number 6 pins. 1 pin will be costly be forewarned.
If Gary is out of them and you dont wish to wait or buy from MC I have a bag of them in the shop and might be talked out of one or two.