Found this on the ULEZ web site.
"Retrofit: convert to Liquid Petroleum Gas (LPG)
If your vehicle has been converted to run on LPG you may meet the emissions standards. Converted vehicles will be treated based on the DVLA's records of its emissions standards. This means diesel vehicles should be converted to use LPG in a spark ignition engine, and the DVLA notified of the change.
We recommend using a UK LPG-approved installer. Our LoCITY programme has an up to date map showing LPG refuelling infrastructure in and around London."
The question is would you really want to drive a 126 with a gas tank fitted? I certainly wouldn't just from the safety point of view.
Just buy a nice early air cooled car, that is a better option