Yokomo bits are becoming a bit of an issue in the UK, not many places are keeping the stocks that they used to. Fortunately the SD came from a period when Yokomo was well supported for parts here, so there is a bit about.
Demon is about the best on-road internet supplier in the UK now. I see bits on eBay from time to time. You can also order from the various suppliers in HK or even Japan although postage can be a bit much if you just need a diff screw.
Personally, since I am running a TOP car that uses Yokomo diffs, I get parts through my sponsor who sells direct now at
www.rcracinguk.com.
As far as the timing speedos go, there are a lot on the market that can have the software updated. I would only buy one that can be updated via USB. That being said, I thought Ardent were running 13.5 untimed as one of their classes. The speedo nonsense is doing a lot of damage to on-road right now, you are not the first person I have heard from who is put off by the massive speed difference. Personally I don't run the timing software which is fine on the smaller club tracks (probably faster due to the smoother powerband) but might be a problem at the big ones (where you get caught very quickly at the end of the straights - the time loss is not so great, but the risk of shunts is high).