Not a specific answer, but the original starter will have been a Lucas M45, M50 or M127 type, however, more modern gear reduction starters are vastly superior.
Apart from being higher torque, the reduction drive style are half the weight and half the volume of the direct drive starter which makes it so much easier to install. As you'll discover, it's a snug fit!
The slim body also allows direct access with a socket improving installation.
This is a good description of the two types
https://www.emmarkuk.com/gear-reduction-starter-motorsEdit:
I should add - I'm still loving the 1-second starts!