Floor Rot

I did some sifting through the FAQ's but didn't find anything. A lot of the older XJ's for sale around New England, like 96 and older, have the infamous rotted floor. New England is not kind to its vehicles in this sense. Seeing as how inspections in RI are performed by what remains of Hitler's Germany, this is a problem that would have to be addressed. Are there any ways to fix that can be done by a novice at home vs. bringing it to a professional welder? Any help is appreciated.

Grinder, reciprocating saw, drill, sheet metal/aluminum or preformed panels, screws/rivets/welder, safety glasses, gloves, sealer.

It doesn't have to look pretty.