I often just marinate chain and cog (if steel), eliminate any stiff links. PB blaster the chainring bolts, and crankbolts to get them marinating. If really bad, a couple of applications on consecutive days may be in order. Measure chain to check if replacement needed, if OK, and no deep pitting, only replace for aesthetic reasons. Repack hub, headset and bottom bracket bearings (assuming cup and cone bearings).
Oh, I see you mentioned cassette. Is it infact a single speed cassette, or a singlespeed screw-on freewheel. I'd just replace the freewheel. Beware of some lowend BMX freewheels, as they can be difficult to remove once installed.