The first thing that you should keep in mind is a big issue that affects basically all 035Ti tweeters at this point. Underneath the tweeter dome there is supposed to be a circular foam plug that pushes up against the bottom of the tweeter dome. This serves to dampen the tweeter a bit and stop it from becoming too bright and harsh/metallic sounding. While environmental conditions can affect just how quickly this foam disintegrates (same as woofer surround foam), by this point I seriously doubt there are any 035Ti tweeters remaining that don't have rotted foam under the dome. It is possible to replace the foam, but it is a very delicate procedure. Even if you replaced the broken 035Ti, you would still want to replace the foam on the replacement.
I strongly recommend against using the Simply Speakers tweeters as anything other than an absolute last resort. They are nothing special, just cheap Chinese tweeters that fit the hole. It won't be a vintage JBL anymore, it will be a hodge-podge Frankenstein speaker; but better than a speaker without a tweeter, I guess...
I would try to do the foam repair on the tweeters that you already have, since you don't have much to lose. Chances are good that during the repair, you might also be able to identify why that one 035Ti stopped working. There are very fragile wires that go from the spades that the internal speaker wire connects to to the voice coil. Simply wiggling the spade connector as you disconnect the tweeter can be enough to break the wire. It is possible to repair the wire though. You might try taking apart the 035ti that is already broken first, to both see if you can repair it and also get an idea if the foam replacement is something you want to also tackle on the working tweeter, before moving forward.
If you are not up to the repair, then I would try to part the L80T speakers out, sell them as-is, or maybe even put the simply-speakers tweeters in them and then sell them (with appropriate disclaimer of course). Use the money to buy something like a pair of JBL 590s. The L80T is not one of the better vintage models, so I would not dump too much money into the project.
