Pro tip: get a new HG51 cassette, then get a discarded used HG70/71 8 sp. one. Screw apart the old HG70/71 8 sp. cassette block using the 3 tiny Allen bolts from the back of the cassete. The spacers are made of metal in those cassettes.
Now comes the brutal part: take apart the HG51 cassette using a hacksaw, cutting the metal rod(s) inside the spacers, which are made of plastic anyway.
Assemble HG51 cassette with the old metal spacers and voila', you will have a better (more retro) looking full metal cassette, which is much more stiff/rigid due the metal spacers.
Downside: the cogs are not screwed/riveted together anymore, so there can be a bit of play back&forth, but if the locknut is properly tightened, and the connecting surface of the freewheel body isn't damaged, this shouldn't be a problem.
Disclaimer: Before doing anything! Always check the width of the metal cogs and the replaced spacers too, before disassembly, because only the same width parts can be replaced with each other to avoid bad shifting quality!
IMO HG51s are okay, if a bit too flexy sideways. Sunrace cassettes can be taken apart properly using bolts, not rivets.