Some metal filled epoxy (like JBWeld) is good up to about 250-300 degrees F. practically speaking. The company might claim 500 F but i know better.. and i don't think they would back you up using it for this particular application..
Epoxy doesn't suddenly fail at some set temperature. It gradually gets soft and decomposes (you can smell it) and looses adhesion and finally fails. Since this is the cylinder casting those temps are easily reached.
I might try a blob of high-temp silicone in that little gap after roughing up the surface a little. Silicone is good up to 600+ degrees F so heat is no problem. If silicone has a problem here it would be constant exposure to gasoline which is not recommended. Since this is not constant exposure and the gasoline is a vapor, i feel better about silicone than about any sort of epoxy. I guess the silicone plug could blow out under pressure? Future disassembly might require another silicone treatment? I would know in a day if this idea was gonna work or not.
First thing i thought of was take it to a good welding shop that normally does aluminum and pay for 2 minutes worth of TIG work.. Take it home and clean up the weld and you are done.. A permanant fix. (If they caught the defect at the factory, this is what they woulda done before shipping it out.)
So my choice #1 is drop by a couple welders for estimates. I wouldn't be suprised if the guy took pity on me and my moped and took me in back and let me watch as they did it for free..
I think you got real lucky and this is easily repairable and spending a couple bucks to have it done right is justified, imo.