Another stripped thread Will? Perhaps your mechanic should stick to tractors. <g>
I agree with others that forcing on oversized bolt, (even one that has been grooved with a flute or two,) into the hole is a bodge more suited to your mechanic, and would in any case push swarf into the sump, but unlike Gareth I would happily use a proper tap. However to do the job I would want the sump removed and mounted on a drill or mill table to get the correct angle, then cleaned thoroughly of swarf before fitting to the engine. I doubt that would be easier or even necessarily cheaper than buying a secondhand sump from a breakers yard.