You can review books going back over a century, such as Audel's Gas Engine Manual, published in 1908, and they all state the same thing; that diesel engines will operate on gaseous, liquid, and solid fuels. "hydrocarbons are hydrocarbons"
The rest of it is just proper filtration, and/or heating, and/or treating, and/or dilution.
Yes, you probably need a certain amount of diesel for combustion (idle) fuel, and will only get a portion of full power on fuels which offer less than what fuel oil (diesel) has, but if the gas is introduced into the combustion cycle (air intake), it will replace some of the diesel fuel.
An engine which is well-governed, such a a Lister, should be near ideal for such a project. The diesel supply is left on, and the engine loaded. Gas is introduced up to the point of "problems" (such as lack of oxygen, etc) which should be around 25-30% of the fuel load, and then the gas "backed off" 5-10% or so.
At that point the engine will maintain smooth operation and at a constant RPM, if the gas supply is constant and of uniform quality.
Have done this with propane, but not wood gas or other producer gases.
Have to think also that the 1920s CS Lister design is much closer to what those early pioneers wrote about, than a "modern" diesel engine running 20,000 psi+ injection pressures.