Methanol is probably the easiest burnable fuel to produce that is also sustainable.
No it isn't.
The only sustainable fuels that can scale to the needed amounts are invariably based on wind/solar electricity that is used to split water into oxygen and hydrogen in electrolysis cells. The easiest fuel to produce that is sustainable is hydrogen.
The second easiest fuel to produce that may be sustainable (if you have a source of enough sustainable CO2) is methane. To produce methane, you need to combine carbon dioxide and hydrogen in a Sabatier reactor. The downside of methane is that burning it releases carbon dioxide, so to make it truly sustainable you need to get that CO2 from a sustainable source. The problem? If you capture all CO2 emitted by the factories of the forestry industry (producing pulp, paper, etc), you will quickly see that the amount of CO2 captured isn't enough to make enough methane to cover all energy uses of oil and natural gas today.
So methane is sustainable if you limit its use to only some critical areas like air and naval traffic. Usage in road transportation is already too much, the amount of sustainable CO2 is not enough to produce enough methanol to be used in road transportation.
Producing methanol and other liquid hydrocarbons requires even more steps than producing methane or hydrogen. The cost to produce these sustainably is probably so high that the market isn't willing to pay the cost, instead preferring hydrogen or direct electricity and in some limited cases methane.
Does an internal combustion engine (ICE) electrical generator that uses methanol as fuel exist?
Oh yes. Large internal combustion engines can burn any combustible liquid or gas. Gasoline, methanol, ethanol, diesel, heavy fuel oil, methane, hydrogen.