Having a think about your loud 'knock' issue.
I've done a bit of digging into the fuel factor and can offer the following.
Excessive diesel knock can be attributable to poor fuel quality, diesel fuel has a 'cetane' rating associated with grade (not the same as octane for a petrol engine); in the UK regular pump diesel has a cetane rating of 51 and this is quite high, we can also get 55 cetane grade fuel at a premium price.
The cetane rating is the delay in ignition after injection, the higher the value the quicker it ignites, your motor is rated at about 20:1 compression so you should get a very rapid burn compared to say a Lombardini that manages only 18:1, the higher the compression the more effecient the motor. In a low cetane fuel there is a longer delay before the fuel ignites but then a much faster burn rate once it has started.
If you were to obtain a higher cetane fuel this should allow you to retard the ignition timing slightly and thus reduce the diesel knock you are suffering.
One of the cost cutting elements of the small stationary diesel motors is the fixed timing, in automotive motors there is an additional element to the injection pump that advances and retards the injection timing relative to the rpm of the motor. This is not an easy fix for us experimenters as the timing is controlled by spacers under the injection pump, less spacers = more advance. In order to effect variable timing you need either to have control over the pump height or better have an advance mechanism for the pump operating cam lobe. In the Cushman scooter world there is a big maket in aftermarket fitting of Briggs & Stratton V twin petrol engines, one of their 'hop up' tuning tips is to fit a stepped drive key to advance the spark. In general terms the more advance you can run before detonation kicks in the more power you can make, conversly the more retatard you can run at lower rpm the more torque you will get. (up to a limit).
LPG fumigation works because you are introducing a secondary fuel that burns at a different rate so not all your fuel is burned in one 'hit', later stationary engines such as the Lombardini 15LD series and the later Hatz models have dual stage injection, this spreads the burn over a longer period and again reduces knock.
I'm not sure if you can upgrade to dual stage injection by swapping injectors only, these work by 'breaking' at a lower pressure for the primary injection followed by a full inject at a higher break pressure slightly after, you may need to match pump and injector as a new set. I've certainly noted that the newer Hatz and Lombardini engines have considerably less knock at idle - Stuart has commented on this before when comparing his earlier Hatz IB30 to the later IB40 motors.
One other factor is your motor has an alloy barrel with a cast iron liner, older motors have a complete cast iron barrel and these are known to be quieter. Not much you can do about this short of knocking up a soundproof casing for the entire engine - not impossible given that you have retained the fan cooling.
So - try a better fuel if you can get your hands on it and muck about with the timing again; the only downside would be fuel availability on a longer ride.
As an aside do you know that diesel fuel is 18% heavier than petrol? fill up an Enfield tank with 14 litres and it will weigh 4lb more than if it were petrol. Up this to one of the aftermarket tanks of 18 or 21 litres and you are looking at carting around an extra 11 or whopping 18lb extra!
In aviation fuel this is critical as the energy content is more or less directly linked to the weight; imagine tanking up your 747 in St Lucia and your tanks may be full but the total weight will be less than if you had refuelled in Iceland; and your range suffers as a consequence. As my grandpapy used to say - fuel up in the mornin' and you get more bang for your buck; I think this was because the old pumps used to store a head of fuel in a glass container at the top, once the sun hits this expect it to expand rapidly. I for one have filled up only to have the fuel expand as the hot engine underneath goes to work.
lol
). At idle and while on the center stand, my bikes front wheel bounces up and down and the center stand grinds into the concrete quite nicely 
but what is gone, is gone forever....