Welcome to BLF Melon.
There aren’t many upgrade kits left for the Maglite Mini-AA but if you’d want to upgrade yours for sentimental reason there is one pretty good looking kit still available from Litt Industries in the US.
They have a LED & Tailswitch combination that’s a simple drop-in.
Although with the added cost of postage from US to Poland it wouldn’t make economic sense when there are much better value lights like the recently released Sofirn SP10Pro.