Thanks for the suggestions darwinmauser and Story. I'm going to start collecting parts and pieces but will resist the temptation to start filing just yet.

The cocking piece can be welded and reshaped where it was ground I think, so probably I won't buy another one.

I'm thinking that thin carbine barrel could do with a nice full length rib to add some strength, perhaps leaving the rear sight as it is and just ribbing above and below it. I'm not a big fan of open sights vs. peep sights, and the holes suggest a vintage scope of some kind (my particular interest!), but I'd need to find out what kind of base was on there first.

I also prefer the ten round magazine, although they seem to be very rare on Lee Speed type rifles; I'm not sure why. It would be nice to get one with the proper chain etc. though.

I've got plenty of No4 stocks, but I like cutoffs too! I know they're impracticable, but they're part of the rifle I feel.