The speakers in question are Mission 770, that I picked up for a song
Jeeez, I thought you were referring to Martin Logan (mentioned in yr "system")!!
Of COURSE you can play around. Hi Vi & B&G could be (cheap) contenders -- for a more expensive (real) ribbon you can check out offerings at Madisound & Zalytron.
You must find the xover's hi-pass frequency & filter slope & the present tweets' impedance (at the xover frequency -- or the nominal one).
The most difficult part will probably be fitting the new tweet to the opening...
