Even if the genny isn't designed for it, you can use the equivalent of a welders exhaust; ducting placed near the exhaust and a fan to draw the fumes out and exhaust them outside. Link it to the supplied transfer block so it only runs when the genny does, potentially automatically as well. Remember, there is also soot, water vapor, etc in the exhaust. You really don't want these to build up in the shed either, especially if you are storing other things like camping gear.

Regards,
Bill