A coolant recovery system is a good thing, but you don't need one. Indeed, 1200s did not come with one. A15s do not need one either. Unless you are using a near-boiling-point thermostat, which is not recommended.

Without it your engine should still cool properly. The question here is, when your engine overheats, is the coolant level low? Say you fill up the radiator, then drive it for a while, when it gets hot (before it boils over) stop the engine, let it cool down, then check the coolant level. If it is still full, the problem is not a bad cap. On the other hand if it is low, it's leaking out somewhere. The cap seal could be bad, a hose clamp could be leaking, it could be pulled into the cylinders (bad head gasket), etc. A pressure test is called for, which any shop can do. Also I would do a compression test just in case. Stop guessing and buying parts you may not need, and start measuring ...