[Veselin Kolev]

Well it all depends on what you want to do. 3 hp isnt that much power. its about 2250 watts. The welders I have are about... 240 KW. I'm serious. 240 volts, 100 amps. Ok and then the small welder I have, a cheap $2000 miller is like, 5 KW. But still, thats twice the power of your engine, and I doubt it still gives 3hp, as its probably old. You can weld steel sure, but it'll be slow. I wouldn't use that motor, unless you seriously dont care how good a welder you get out of it, and just want to do it for the point of making a welder out of a gas engine. I always appreciate those kinds of efforts.

As for the alternator question, generally welding runs on low voltage high current, so I would go with something around at least 100 amps for the 3HP engine, but less than maybe 160 amps. Also remember that the alternator gives 100 amps at its nominal speed, meaning you have to gear it off the lawnmower. Then I would get some nice cables for that thing, you dont want to blow anything (or anyone) up with that much current. I wish you the best, and good luck.