Synthesizing hydrocarbon analogues of fossil fuels (petrol, kerosene, diesel, bunker fuel[1]) is possible and has been theoretically demonstrated.[2] The problem is that it's not economically feasible, in large part due to structural market failures in the price of petroleum.[3]
Physical abundance of the constituent elements has little to do with production costs of the resultant fuel.
And hydrocarbons are vastly preferable to ammonia as fuels for all kinds of reasons: energy density, noncorrosive nature,[4] non-toxicity, convenience in general handling and storage, etc., etc., etc. So long as you're synthesizing fuels, make it the good stuff, not poison.
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Notes:
1. That last is probably a non-starter. It's harder to synthesize longer-chain hydrocarbons, so far as I'm aware, and the primary driver of bunker oil for marine propulsion is that it's an otherwise low-value surplus from conventional petroleum production, even a large fraction of much extraction, e.g., Venezuela's very tarry petroleum,and Canada's tar sands. Lighter fractions would be easier to synthesize and more attractive as fuels.
2. For about 60 years, including research at Brookhaven National Labs, M.I.T., and the US Naval Research Lab, as well as with a Google moonshot project. A list of sources is here: <https://news.ycombinator.com/item?id=28970111>
3. A deep topic, but given the fact that we're extracting petroleum at roughly 1 million times its rate of formation, and in a highly unsustainable fashion, there's a fair argument that petroleum ought to be priced about 1 million times its current market price. The economics of nonrenewable resource extraction is grossly irrational and divorced from physical and geological realities. On rates of formation, Jeffrey S. Dukes, "Burning Buried Sunshine" (2003) <https://core.ac.uk/download/pdf/5212176.pdf> (PDF). Previous discussions: <https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...>
4. Indeed hydrocarbons are routinely used as lubricants and protectives for metals and other materials.