I am (hopefully) a couple months away from finishing my project which uses the same BMW gearing as in your Dnepr. I purchased a 5 percent taller 5th gear for the gearbox from Siebenrock in Germany which should increase calculated top speed to 67 mph @ 3600 rpm. The gear, which has 20 teeth instead of 21, was made by Wolfgang Kayser who apparently retired a year or so ago. He must have made a batch of them as the gear is still listed by several companies including Sibenrock Note that BMW changed the gear angle sometime in late 1981 and you have to verify which gear you have (later gears marked with an X).. In some cases two gears are required.
Kayser also made a 10 percent taller 5th gear set (two gears) good for 70 mph. David Hubbard's BMW R80RT has one (see
http://www.dieselbike.net/bmw.htm) I haven't found anyone selling this gearset.
Alan Forbes of Indian motorcycle restoration fame on West Scotland Lane, Edinburgh, Scotland is (was?) building a modern interpretation of the Indian Four based on the Swedish Wiking. The bike used a BMW 5 speed gearbox and BMW rear drive. Kayser initially made a special taller gearset for these bikes. In an email exchange with Alan several years ago he mentioned he was about to make his own gears in house.
Perhaps someone in the UK could follow up on this. I don't know if Alan is still building the bikes but might possibly have some gears that he would sell which would help us greatly.
I am using a Daihatsu D950 which is about double the horsepower of the Lombardini LDW 502. Since I haven't been able to find any posted performance data for a diesel/BMW setup I've spent many hours number crunching and have determined that I can probably run 3.36:1 overall gearing (with a modified 4-speed gearbox) for 55 mph @ 2400, 70 mph @ 3000, a top speed of 80 mph @ 3600 rpm. *However*, consulting the engine's hp/torque graphs and hp vs mph requirements, there would be minimal roll on acceleration; frequent downshifting would likely be necessary. I do intend to eventually try the setup just to satisfy my curiousity. Initially I will be using a stock 5-speed gearbox which should provide very good performance.
For your convenience, here is a simple formula for calculating speed/rpm for BMW and similar bikes:
mph = rpm / (gearbox_ratio*rear_drive_ratio) * tire_circumference / 1056
example:
rpm = 3600
gearbox ratio = 1.5, rear drive ratio = 2.91
400 X 18 tire circumference = 81.68 inches
3600 / (1.5*2.91) * 81.68 / 1056 = 63.79 mph
Hope this helps.
The story about why it was dismantled is that it had a blown gasket, so hopefully that is all that's wrong and there will be no nasty surprises.
But without knowing the economy of other Diesel bikes I don't know if I've got overly high expectations.