Hi Kirley, that's me! The reason the thread didn't continue is that I gave up on the Silverfox bodies when I couldn't get the cabs to look right.
You're right, it's an expensive option. In fact, I hadn't intended on getting the motor at that point... I was exploring the possibility and contacted SB Modellbau to see if any of their products suited a Lima Class 31 chassis. They weren't sure, so they asked if I could send it over for them to measure. The next thing I know, I got an email saying the motor was installed and would I please send on the money!
I've only tested it on level track, but it's a lovely smooth runner, and quiet too... it glides along! And the brass motor bogie is robust, to say the least. I believe the Maxon motor installed in it is also used in surgical instruments.
If you want, I can post it up to you so you can test it for yourself...