Hi Bill, I should think that sawing will certainly work, especially since the shaft is welded so it won't spin on you. SawzAll with a metal blade may be easier than a sabre saw, but either should do. Seems, though, that you'll still have the issue of drilling out the weldment.
Also, I suspect the original installation was welded purposely so all of the frictional forces were distributed across the full width of the roller's internal bushing. Replacing with a bolt will likely allow the shaft (new bolt) to turn rather than just the roller/bushing which may cause undue wear at both ends where there's a point load against the support brackets. With some salt crud build up I could see the ends of the bolt wearing through in (relatively) short order. You may want to weld the new bolt in place (just like the Captain did! :-) .
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Cheers, Craig - SN68 Sangaris, Tropic Isle Harbor, FL