A quick google found this dated 21/05/2021, suggests that funding has been made available, just ned to find some not too old vehicles to spend it on!
"Launch of Clean Air Bus Fund gives Bradford buses £1.6 million boost
Article dated Friday May 21, 2021 at 12:11 PM
Bus companies providing services in the Bradford district are being invited to apply for grants to make their vehicles cleaner and produce less pollution.
Bradford Council is Launching the Clean Air Bus Fund today (20 May, 2021) providing £1.632m worth of grants to help businesses meet Clean Air Zone standards ahead of its launch in the district in January 2022.
There are thought to be around 100 buses that currently do not comply with the Clean Air Zone standards - which mean they emit higher levels of Nitrogen than those with a Euro 6 diesel engine or ones that run on alternative fuels.
Grants of up to £16,000 will help operators to replace or retrofit their buses with equipment to reduce the pollution they emit. Funding is available for both scheduled and tendered services".