Hi. OK, a MK1 Impact, are you talking a smooth twist ( without the X barrel option ) - if so, I am unsure that a kit is available for a straight liner swap. You may need a barrel kit, but don't hold me to that.
The original plenum, I would guess not, you could make a lot of other changes, heavy hammer or do some drilling ( which I would never advise ) and still not get to what you need. I do think the quick and simple way is to just change the plenum over. The kit does work on all Impacts. Only question is then which barrel do you have, If you shoot Hybrids the normal X Barrel will be sufficient - however if you want the ultimate then yes a slug liner, and if you want to shoot other brand slugs a liner is advised. Hope that kind of makes sense.

Ah yes! It is a smooth twist, and that would make sense as the threaded insert that keeps the liner in doesn’t have the spanner groves to remove it. I will be using hybrids when you have them in stock for .25 as I’m also in the UK and it seems that they would be my most sensible option as Nielsen slugs are non existent over here, so yes I will get a smooth twist X.
On that note, where might I get a power plenum from in the uk? I have been looking and I am still none the wiser!

I don't think the kit is here yet. I can ask and find out, give me a day or two. Will come back to you.

Thank you very much

Do you still need a power plenum?
I would have a complete one in mint condition left....