That sucks...door stop.
I checked my MX-80...and it seems to default to the individual input setting....and then there is a small switch that let's you set the lower row to either Sel-Rep or Repro. Normally I have that option set to Sel-Rep for the lower row of channel switches...I mean, there's little reason when you would want to have individual Repro switches during normal tracking use...so odds are, if the remote died, I would be stuck in the Sel-Rep mode. You can do playback with that but it's coming off the Rec head, not the PB head.
That's my 124 remote.
Considering the value of having the remote...with these old, out of production decks, there value in having a spare too..
I also have the later 140 remote...but I didn't pull it out to see how it defaults, though odds are it is the same as the 124.
Well...on the "positive" side...you might barter for the MS-16 anyway, and talk him way down in price, considering it's "useless" without the remote (that's how you bargain with him). Then keep hunting for a complete MS-16, and you have spare parts machine ready to go.
Yeah, it's a gamble...and who knows how long before you find a complete MS-16 ....but, just a thought if you're really committed to an MS-16.
If ever see an MX-80 semi-locally with or without remote for like real cheap...I would consider getting it just for the spare parts, though I've already accumulated quite a lot of spare stuff already.