Starter motor issues.

Good afternoon guys,
My first post in the series forum although I have posted a fair amount in the Range Rover section. I am most of the way through the rebuild of my 1967 SIIa 88" that has a 3.5 V8 engine conversion (done in it's past before my ownership). I am having a bit of trouble with the starter motor (I think). The starter motor seems to be a later SIII petrol starter as it's not a pre-engaged one (I think it's an M418G model). When I try to start it the engine turns over sluggishly a couple of times and then stops, very similar to if the battery was dead. The battery is brand new and fully charged, I even tried a jump pack to help boost it but still the same. I took al the plugs out and it spun it over when there was no compression. I put the plugs back in and it was the same, turning over sluggishly a couple of times and then nothing. Checked the earth and it seems good and it has a new power cable from the battery. I have taken the starter motor off and manually tried it with jump leads, it fires up but in my opinion seems a bit sluggish and doesn't zip up as I'd expect. Could it be a case of it needing new bushes, new brushes and the armature cleaning up? Any help would be greatl appreciated!
Andy.

Yes, quite probably. It's an early starter so it'll be quite old - might be worth getting the proper one, the 3M100.

Cheers mate. The engine is from a Rover P5 as far as I can tell and the conversion is many, many years old, will this starter still mesh correctly with the fly wheel with the 3M100 Being for the EFi engines, this being the 3.5 twin SU carb version.
Cheers again,
Andy.

I just checked over the starter motor and found the following;
Lucas
26266F 53 71
M45G.

That's the pre engage one off the diesel 2 1/4. The 3M100 is in the S3 parts list as the original V8 fitment so it should work.