Can you provide a close-up photo of the bits ?
Throw them away, very small metal flakes
Good to hear that things now seem to be OK; but I am surprised that 'bits' as small as that, jammed your valve cup.
Rorf gives good advice. Fortunately, the A series engine will withstand a fair bit of abuse, so may I suggest that you fit an oil gauge as this will tell you how much pressure your pump is producing.
Some people like their Mini pressure relief valve to blow at 80 PSI, because, they reason, besides lubricating the bearings, the passing oil carries away a lot of heat, but ironically, that is just what is happening in a well worn engine with only 20 PSI, and amazingly, they hang on for years and outlast the body in damp, road-salted UK. Things are probably very different in your part of the world, but if you get the chance, buy a spare engine and do one up and have fun modifying the other.