It's an interesting question because I think Pappy and others instinctively know that a round brass fitting like that will not crimp onto a round spring wire with any degree of permanence. It will simply fall off.... I did some investigating...
Note the photos here:
https://www.ebay.com/itm/Force-US-Marine-Chrysler-6-9-9-15-hp-Outboard-Recoil-Starter-Spring-FA286970/171227251546?epid=1444761095&hash=item27ddf0835a:g:z30AAOxyn9BRcQdk:rk:3
f:1&frcectupt=true
Does the end of the spring have a flattened or notched surface ? If not it might be one that was broken and somebody repaired by bending down more of the round wire.
If not I'd suggest you create a notch or barb on the spring end, then crimp the brass over that. just crimping the brass over a round spring wire will insure it falls off after a very short time.
Heat it to remove the temper then tap it flat. Then grind barbs that the brass can deform over.