Great news! The fusible links can easily be tested by trying to pull apart the insulation, if the wire inside is broken you'll see it stretches out when you do....or just an ohm test at either end to see if it's open circuited.
Read the earlier posts again about using Mega Fuses, and/or fused Junction block options near the battery or at the battery and take both of those 12 ga RED wires that were using the fusible links out of the engine harness, and try to bring them up from the starter motor solenoid to the top side of the engine, or fenderwell. If needed get yourself some quality 12ga (preferably red) wire and extend them both to the junction block, or directly to the battery. At the junction block, or battery post put some new fusible inks (like was originally down by the starter). If you put in mega fuses, I would probably go with about 50A, maybe 60A for each of those two RED wires.
You DEFINITELY do not want those two circuits left connected to the battery/starter post without any fusible link, or fuse protection. So even if you leave them down by the starter solenoid post, make sure you put back fusible links again. If not a short in either of those big red wires could cause a fire.
Given they're 12ga, you should be able to use something like this (7ft roll) and make your own fusible links, just get some good quality crimp and heat shrink sealing ring and butt splice connectors.
You can buy also them pre-made (with ring terminal and butt splice) as well from lots of online vendors, or local auto parts, Dorman, etc...it's really up to you.
I wouldn't use any cheap inline ATO style fuse holders in place of fusible links, especially if you leave them down by the starter solenoid where there is heat and usually oil/fluid leaks.
Good luck, sounds like you just need to make the permanent repair, whichever route you decide to go.
Here's a little more service manual information for you as well.
