I use Pro-Weld, which is probably like JB-weld, cold welding poly compound.
For a hole in the base, if the mower is steel I weld it up as best as possible, then fill with poly-weld, then add some bog, then sand off until smooth. If the base is alloy, I drill holes, then rivet an aluminium plate over the hole, then JB weld and bog, then sand off once dry. In most cases the damage cannot be easily spotted, especially once it is primed and painted over.
Mowerfreak, I have successfully rebuilt fins using pieces of folded aluminium and Pro-Weld formula. Works a treat, but you have to have the area clean as a whistle before applying. It can be very fiddly. I am constantly amazed at the strength of this stuff. Never as strong as the original of course, but in some cases close to it.
