I have a saws-all but I think the blade is too big to make a circular cut and with the handle around the top, I don't think my jig saw will do the trick
you may want to use the search function, I just searched for "cut the top of keg" and there are lots of great posts and ideas. Here is one you should read thru showing the use of an angle grinder and on page two a video showing a jig that someone made. https://www.homebrewtalk.com/f51/cutting-top-off-sanke-keg-using-angle-grinder-88907/
also if you have a friend with a plasma cutter those are AWESOME to use for such a task..
either plasma cutter or angle grinder. You can pick up an angle grinder and the cutting wheels (not grinding wheels) at harbor freight for less than $50. I would also recommend getting a flap wheel for dressing the cut edge.
Tried using a grinder and burned the motor out on it (probably trying to make too deep a cut). Wound up using my reciprocating saw with a metal cutting (18 tpi) blade to cut 2 tops out. If you have a bimetal hole saw (3/4") use that to create a starter hole. I used the rim of the keg as a guide for the housing of my recip saw.
I just got done with mine using an angle grinder and jig similar to the "Keg to Kettle" conversion youtube video. it took about 20 mins for the first one and 10 mins for the second. But they came out perfect. Now I need to put holes for the fittings into the sides of the keg.