Hi Paul,
My finished Peaks are all running with the older Bachmann chassis, which is ok but not quite as good as the new ones. Llanbourne has 45 029, 45036, 45103, 45111, 45141 and 45150. 45036 and 45150 are Mainline bodys and 45111 is a Mainline body with A1 etched nose grilles, this didn't work as well as I had hoped so I didn't do any more.
I am not sure if the Replica or early Bachmann body would go on the new chassis. The problem with the Replica and early Bachmann is that the nose is a bit too low and to make a 45/1 you need to alter the roof which isn't too hard, raising the nose can be done and I have had a go, it's a bit of a job. Something to also remember is that 45/1s don't have any side steps each side at the boiler end were as the 45/0s still had them but plated over.
I have one of Bachmann's new Peaks in Green liv, the plan is to do another 45/1, the nose on the new models looks alot better but the nose grilles as all their releases has the old type, it's a shame as the early Bachmann model had the right ones. Roof mod and side steps will need to be sorted along with the nose grilles on the green one so chances are Bachmann will release a 45/1 when I have finished mine.
Hope thats is of some use for you.
Cheers Peter.