The guide groove is there to stop the piston rotating but if damaged can cause the piston not to rise and fall smoothly. File with care. The piece of folded metal may well have been causing the fuel flow to be biased into one of that pair of intakes so there may be an improvement in running with it out of the way. Carbs look as if they need a rebuild but the essential thing to look at is how worn the throttle shaft bushes are. If there is any wear at all they really need replacing along with the shafts if you are to get good idle and low speed running, as the air leaks make tuning impossible.
If your engine is a B20 and the breather pipework is all correct the hose from the filter to the cap will be flowing air from the filter into the engine so there should be no problem with oil coming into the engine via the filters. Carb cleaner or cellulose thinners plus an old toothbrush should make the carbs look a lot better. A basic gasket service kit isn't expensive.
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