option 1 -Get an extension made with properly crimped eye lugs. Connect the original and the extension using a short bolt an nut, with flat washers and a lockwasher. The terminals should touch, no washer between them. Cleaning any corrosion off the ends is a good plan. Wrap with self vulcanizing tape and electrical tape.
option 2 - Have a new cable made. I have never seen a starter motor where the wire goes direct inside somewhere, but I suppose they exist? Check and make sure there's no stud connection.
option 3 - extend the cable with a butt splice. This is most difficult because the original cable is a metric size that is not equal to any AWG size. The butt has to be sized properly to crimp properly. The factory cable is close to either #8 or #6, I don't recall which. Use heat shrink (glue coated is best) over the joint.
Regardless - DO NOT use solid wire. It belongs inside walls where nothing moves. It should never be used on any vehicle except to tie a loose muffler up until you get it properly fixed.
Crimping - small crimp lugs can be fairly successfully attached with hardware store crimping pliers. There are proper tools to do this right, but they cost a bunch. For larger wire sizes, you really need the proper tool. It is not worth buying it for a couple of crimps. Find an electrical contractor that can make up the cable.
The best cable to use is welding cable, this has very thick insulation though. Car stereo shops have quite flexible cable that would be good. Standard electrical wire with 7 strands is still about as stiff as a coat hanger wire and a true pain to bend and route: it's made to be pulled into large conduits with large radius elbows - pulled with a winch. You want wire that's much more flexible.
