There are after market ones available. MinnKota used to make one but it was discontinued. However a member recently posted one that he had purchased and put on his boat. Here are some threads I found by searching for "variable control".
https://www.crappie.com/crappie/main...speed-control/
Also we have a MinnKota authorized repair shop here named "The Crappie Hole" Mark is the owner and I understand he makes a variable speed unit. Here is contact info:
[COLOR=rgba(0, 0, 0, 0.87)]The Crappie Hole[/COLOR]
Directions
Save
5.02 Google reviews
[COLOR=#878787 !important]Fishing store in Lexington County, South Carolina[/COLOR]
Address: 726 Amicks Ferry Rd, Chapin, SC 29036
Hours: [COLOR=#212121 !important]Open ⋅ Closes 6PM[/COLOR]
[COLOR=#212121 !important]
[/COLOR]
Phone: (803) 345-5606
Mark 1:17 ...I will make you fishers of men