Is there a formula to convert SFC to air-fuel equivalence ratio λ?
No, there is no formula to calculate the air-fuel equivalence ratio from specific fuel consumption (SFC). To calculate the equivalence ratio, you need the air mass flow, and the fuel mass flow. From this data, you can calculate the actual AFR (air fuel ratio), and lookup the stoichiometric AFR for your given fuel. 

$AFR = \frac{m_{air}}{m_{fuel}}$

The air-fuel equivalence ratio is then simply calculated from these two AFR values.

Air-fuel equivalnce ratio: $\lambda = \frac{AFR}{AFR_{stoich}}$  

You don't have a value for the air mass flow. You only have fuel flow from the SFC and current power.

If you had the equivalence ratio and SFC at two different operating points, not just one, you could assume linear interpolation, or extrapolation, to the SFC in question. 

With the equivalence ratio and SFC at just one point, you would have to find a characteristic curve for SFC vs equivalence ratio for a similar type of piston engine and the same fuel type, and assume the shape of this curve is applicable to your particular engine.