Hmm... ya might be able to tap - just one size larger is all, sometimes going the next size up using SAE instead of metric will do the trick... but I dislike "hybrids" of standard & metric (not that I've much of a choice w/my build heh).
Another option for an oversize that ya want to use the original size fasteners are Heli-Coil inserts
, they're a bit costly but handy to have... I'd just tap one size up tho 'cause ya hafta w/a Heli anyway.
Welding to the cast aluminum these engines are made of is... dicey. It can be done by a welder w/real skill - but yer runnin' the chance of blowing a big hole in the case and/or havin' beads that just won't hold