Backfiring is "generally" an issue with the ignition portion of the engine. Not always, but that is the first place to look.
Try getting some starting fluid and squirting it into the carb. If the engine starts even for just a second or two, then you have a fuel issue. Bad gas, clogged filter, etc.
If there is no difference after trying the starting fluid, meaning the engine refuses to run, then your problem is probably ignition.
Timing chain - Doubtful
Bad gas - maybe, but it wouldn't have run in the first place
Clogged fuel filter - Probably. If the rig has been sitting awhile, some of the gas may have gelled, broken loose and clogged the filter. (The starting fluid will confirm fuel delivery problems.)
Plugs and wires - Always a good idea to replace on an older vehicle, but to suddenly stop it from running, very doubtful.
Stuck choke - Common problem on 440's. Easy fix. Open the butterfly "flap" on the inlet of the carb if closed shut.
