I investigated ammonia a while back and concluded that it was one of the best options after hydrogen. It will produce a specific impulse comparable to a LOX-LH2 chemical engine.
Regarding the possibility of a dual fuel engine, I've come up with some numbers to ponder. I performed a simple design of a hydrogen engine and then recalculated the numbers with alternate fuels to see how they compare. The design parameters used included a 2,000 megawatt reactor, an operating temperature of 2,500 K, and a 40:1 nozzle expansion ratio. For the first set of calculations, i.e. the hydrogen engine, I used a combustion chamber pressure of 40 atmospheres. Below is what I came up with:
Propellant: Liquid Hydrogen
Propellant flow rate: 50.5 kg/s
Reactor power: 2,000 MW
Combustion chamber temperature: 2,500 K
Combustion chamber pressure: 40.0 atm
Gas molecular weight: 2.012
Specific heat ratio: 1.30
Nozzle throat diameter: 27.63 cm
Nozzle exit diameter: 174.8 cm
Expansion ratio: 40:1
Thrust (vacuum): 434.8 kN
Specific impulse (vacuum): 878 s
Propellant: Liquid Helium
Propellant flow rate: 154.7 kg/s
Reactor power: 2,000 MW
Combustion chamber temperature: 2,500 K
Combustion chamber pressure: 79.9 atm
Gas molecular weight: 4.003
Specific heat ratio: 1.67
Nozzle throat diameter: 27.63 cm
Nozzle exit diameter: 174.8 cm
Expansion ratio: 40:1
Thrust (vacuum): 778.5 kN
Specific impulse (vacuum): 513 s
Propellant: Liquid Ammonia
Propellant flow rate: 158.2 kg/s
Reactor power: 2,000 MW
Combustion chamber temperature: 2,500 K
Combustion chamber pressure: 61.0 atm
Gas molecular weight: 8.505
Specific heat ratio: 1.30
Nozzle throat diameter: 27.63 cm
Nozzle exit diameter: 174.8 cm
Expansion ratio: 40:1
Thrust (vacuum): 664.0 kN
Specific impulse (vacuum): 428 s
Propellant: Liquid Water
Propellant flow rate: 245.5 kg/s
Reactor power: 2,000 MW
Combustion chamber temperature: 2,500 K
Combustion chamber pressure: 67.5 atm
Gas molecular weight: 17.884
Specific heat ratio: 1.18
Nozzle throat diameter: 27.63 cm
Nozzle exit diameter: 174.8 cm
Expansion ratio: 40:1
Thrust (vacuum): 779.8 kN
Specific impulse (vacuum): 324 s
Helium, ammonia, and water all produce higher thrust because the propellant flow rates are much higher than hydrogen, however hydrogen has by far the best specific impulse. The high flow rates of helium, ammonia, and water are needed to keep the engine from over heating. The flow rate is the mass of propellant that 2,000 MW can raise to 2,500 K in one second. The combustion chamber pressure is that necessary to force the require gas flow through the nozzle throat (which is of fixed dimension).