Strictly, you need to know RPM, manifold pressure and OAT to calculate power output. Mixture setting is also a variable unless you are consistent with your setting procedure.
For a fixed pitch set-up you will either need to use the POH if it has figures for your propeller (a fine prop will give less power than a cruise prop at the same RPM). Alternatively if you have an MP gauge and an OAT gauge, you can use the standard Lycoming power curves from the operators manual.
A rough rule of thumb for FP is to increase the RPM by 1000 for every 2-3000' increase in altitude to maintain the same cruise power and set mixture accordingly.