While a severe exhaust leak *might* indirectly contribute to reduced engine performance which *could* lead to misfires (if severe enough), that wouldn't be a timing problem itself but a symptom of something else. The timing would remain set as it is, although the misfire could make the engine run poorly and might even affect the engine's ability to produce enough power to reliably maintain the set timing under certain conditions. But the source of the problem is the leak, not the timing itself.
