Hi,
I think you need the complete manual (cable) drive unit, what you have looks like the remains of the electric Speedo drive. I had to do the same thing when I converted to the 5 Speed, I e-mailed one of the LT77 gearbox re-conditioners that appears from time to time on e-bay and he helped me out, for a reasonable price. I then had to change the inner drive gear as it was broken and spinning on the shaft, not an easy job with the gearbox still on the car!!!
Unfortunately these bits seems to be getting hard to find and you just have to take what you can get, RobSport (the TR7 people) had some new bits a while back, might be a contact for S/H as well?
I ended up having to bore out the ID of an earlier inner drive gear (who said these boxes are all the same!!) and fit that (remaking the drive flats in araldite, see pic.) as that was all I could find at short notice.
Mine has black gears and the Speedo reads approx 60% of what it should, some day I'll sort that, for the events I do I use the Rev Counter for accurate speed calcs.
Hope that helps,
Tim