Hi Westernfan,
glad you like the 121.
Westdale do some detailing parts as accessory packs:
windscreen wipers, etched windows, jumper cables, speedo, drivers desk, Oleo buffers, bogie steps (wide and narrow) and seats (double and triple)
and if you speak very nicely to Dick Taylor (Westdale proprietor) he might sell you the whitemetal castings for the chassis as well - he would really like you to buy his complete kit.
As has been noted elsewhere, Dick Taylor will be taking a sabbatical next year, so it may well pay you to contact him sooner rather than later - no website or email, just phone - 01628 482493
The partitions in the Bachmann model are in the wrong place and need repositioning, the partitions behind the driver are missing and Peter Clark does a 1st Gen DMU cab fitting which includes the partition, floor and desk all in one etch + the whitemetal fittings to suit.
I hope that this is of use, please let me know if you need anything else.
cheers
Mike