Thanks, as you (and others elsewhere) suggested I was finally able to get the replacement rollers installed by putting the C-spring on the shaft and wiggling the outer roller over it while using some needle nose pliers and an x-acto knife to compress the ends of the spring. Interestingly, neither the original German nor the replacement outer roller had any apparent radius to assist the installation. The inner roller, yes, to fit over the small ramp where the shaft rises from the bolt head, but not the outer roller.
Be that as it may, all's well and the gun shoots just fine now. Well, except for the very hard trigger pull (it's a TNW-made gun with a single trigger roller).