A few studies from the effects of burning hydrogen as fuel are showing a worrying effect on the upper atmosphere. As we know, burning/combusting hydrogen with oxygen produces water as a by product, all good. But, when you burn/combust hydrogen with air which is made up of oxygen, nitrogen, argon, carbon dioxide, neon, helium, methane, krypton, hydrogen itself, nitrous oxide, xenon, ozone, iodine, carbon monoxide and some ammonia trace compounds it's not so simple. The problem is all sorts of weird oxides & nitrides are created which can do significant damage to the ozone layer. These vary under the conditions they are made such as tempreture & compression and they are very difficult to filter out of the exhaust gasses. Methanol looks the least damaging option and could be used by most diesel engines without much modification. It could be used by aviation too, but it's difficult to see when it's on fire...
Whatever happens, it's not going to be either easy or cheap.