Riptide 70/80 Frankenstein?

I’m new here and don’t know much terminology so bare with me please…. I bought a 70lbs thrust riptide tiller handle style and it worked fine for a while then went out on me, so I took it to 2 shops both of them told me the lower unit was burned up and it wasn’t worth replacing. So I was looking around on Craigslist and found a lower unit and shaft that somebody had taken the top off of and cut the wires, it was $20 so i bought it. I believe it was from an 80lbs thrust riptide with the tiller handle. I was hoping to hook that up to the top that I have from the 70, but the shaft and lower unit from
the 80 only has the red and black wires coming out of the shaft as far as I can tell(I cut the shaft about 6” to see if there were any other wires but I didn’t find any). So I decided to try it anyway, I did a quick test and twisted together the red wires and the black wires and it runs but only at full blast forward and full blast backwards.

Any easy way to get the speed control back? Am I guna kill myself trying to get this to work? Does this make any sense?

Thanks in advance!