The 2 limiting factors are (1) there are no RTR BPRC engines currently available and (2) I haven't found a single source for the components I use.
Here's a diagram that I have used for most of my BPRC'd engines:
The DC motors I used are the ones that came with the engines.
The Deltang RX-65 receiver (and TX-22 transmitter) can be purchased from 1 dealer (in my case I used Micron Radio Control in the UK). The Tx takes a 9v battery. I get my Rx pre-wired so I don't have to touch a soldering iron to the board, just to the ends of the wires.
The 9.6v 2000Mah NiMh battery is made by Tenergy, available on Amazon for under $20. You'll also need a charger. Capable of hundreds of charges the batteries I have (18 engines last count) should last many more years
The BIK-U3 and charging jack came from RCS Australia. Love these things but it takes 10-14 days to get them. Basically the thing can be made using a charging jack, on/off switch, and some polyfuses, but RCS tidies it all up on a small circuit board. I also installed a polyfuse inline between the motor and the Rx.
And you'll need a couple of LEDs for headlight/rear headlight. I got mine from Evans Designs.
Here's a basic diagram with individual components (switch, fuse, charging socket) vice using the BIK-U3:
So far I have installed BPRC in a MTH RailKing Imperial USRA 0-6-0 (smallest engine I have) to a Williams brass N&W J 4-8-4 (biggest/heaviest engine I have). Most have 9.6v 2000Mah NiMh batteries, some have 12v 2000Mah NiMh batteries, and some have 11.1 2200Mah LiPo batteries. All give me runs of 2 to 2.5 hours and take the same time to recharge (haven't tried anything more than 1A/hr charging). All steam engine batteries are in the tenders, all diesels are inside under the shell. I have installed BPRC in MTH, Lionel, Weaver, Atlas, Williams, and Williams by Bachmann engines and all have performed wonderfully.
Bob Walker has provided me with much expertise (thanks Bob!), but a lot of it was on my shoulders. I do (did) have some electronic knowledge from working on Navy gear back in the 70s-80s, but I've been retired almost 14 years and have forgotten most of what (I thought) I knew. If you can follow instructions and solder you're half way there!!!
Go here to see what the BIK-U3 looks like: