Fit is very important for a pleasant and productive shooting experience. Most production firearms are made for the "average sized, right-handed shooter." By and large, you can get by with this.
In my younger days, I would simply remove the cheek rest and re-finish the rifle. As I got older (and had better means) I made the shift over to left-hand stocks and custom rifles.
With shotguns (or fowlers), cast in the stock can be more of a problem with left-handed folks. If the gun has cast-off (for a right-handed shooter) it will lead to improper mounting, and you will miss more than you care to.
Almost all European shotguns have some element of cast-on built into the stock. Fortunately, some offer a left-hand option, which is well worth the extra cost.
Some folks believe in "bending" the stock for cast-on. It is done by a couple of different methods. I don't recommend it as the risk is too high of damaging the finish or worse, cracking or even breaking the stock, especially when dealing with a vintage or antique piece of wood. Even if done successfully, there is the chance of the wood simply shifting back over time.
If you are left-handed, ask questions about the gun, especially if it is a fowler, as you may be fine with the lock on the wrong side, but the cast in the stock may prevent you from shooting it well.