Modding implies improvement, and often minor ones at that. What you want to do is basically take some that works perfectly well (besides the driver which could be easily replaced), and most likely ruin it.
Retrofitting an existing cab to fit different sized drivers (especially drivers that will take up more space than the previous one) is probably at least as difficult as building a cab from scratch, if not moreso.
My guess is that the cab will be too small for the 10s, both in terms of baffle area for the speakers themselves, and in terms of internal volume for the 10s to do their thing.
Having said that, since it's just a cheap combo anyway, it doesn't really matter too much what you do with it. But a lot of guys try to do this nonsense with rare and/or vintage gear where there are a limited and declining number of units out there already. And to me unless you truly know what you are doing and have a clear plan towards making something like that better, you should leave it alone, sell it to someone who wants it for what it is, and then use that money to get what you actually wanted in the first place.
Personally, in this case, I would just replace the dead 15 with a higher quality one (which should take all of 10 minutes, and probably be cheaper than 2 10s depending on what you get) and then sit back and be amazed how much better the thing sounds for so little money. Depending on your taste in tones, you could go for a newer high power Eminence thing, or something like an old EV or JBL. A lot of guys will say it's a "crap shoot" doing this kind of "cab stuffing" as they call it, but in this case, starting with a low end combo that is probably a pretty typical 1x15 size, I'd guess your chances are about 90% that pretty much ANYTHING you put in will be an improvement
