I hate to say it, but, proper smokers, that will maintain an even temp, and not burn charcoal like there's no tomorrow are complicated designs. They look simple but they are not, lots of calculations go into the size and placement of stack, the size of the hole between firebox and smoke box, size of the firebox and its vent, etc.
If the thing was even designed with a vent in the firebox, I doubt all the other calculations were done either, so you might be spending $ to get vent in firebox, only to find out, you got other issues. If you had the equipment to do yourself I would say why not, but if you got to shell out $, not sure it would be worth it.
If you want to "Play with fire", watch Craigslist for a COS (Cheap offset smoker) that is in good shape, lots of info available about simple mods that can turn them into decent smokers. Or a webber kettle, lots of info on way to use them for anything you can imagine.
If you just want a way to make decent smoke, you can usually find a used MES (Masterbuild Electric Smoker) on Craigslist for undere $50, add an
AMNPS and you got one heck of a set it and forget it smoker for well under $100. Lots of info on here for MES as well.
If that thing is decent metal (looks like it is), you could have the firebox cut off and the hole sealed, and have one heck of a nice "keg" grill, that would have lots of grilling space and you could even do small smokes on it via Indirect/2 zone smoking.
Almost forgot to answer your actual question :-). Most common place for the vent is in the door of the fire box.