The fusible link can and does handle 50-60 amps from time to time by original design. No way a 20 amp fuse will hold up.

It sounds like the original problem was fusible looked in poor shape. Suggest you replace with another fusible link. Fusible link is available in 16 gauge (original I believe) or 14 gauge at about any auto parts store.

I had to replace the link in a fairly new premium repop harness because the small spade connector was failing and overheating it. I put the new link (from Autozone) in with soldered ring connectors at each end and a small bolt. Plus electrical tape. Not elegant but no overheating since then and still easy to service if needed.