I touched on this subject in another post. Here the basic information to do such a modification. The diagram shows the basic blocks of a squarewave inverter with the two resonating elements (C1, L1) added. The values are load dependant and the values shown works for a 100W load.
In the first case C1, L1 is switched in circuit producing the waveforms shown. When C1,L1 is taken out by switch S1 the typical squarewave switching waveform returns.
Note that C1 should be non-polarized and L1 should be capable of carring the switching current without saturating.
Average values can be calculated by this:
Xc = E^2/(10^3 x 1.5 x kVA) where kVA is the rated kVA load on the output
C = 10^6/(2 x Pi x f x Xc)
L = 10^6/[4 x (2 x Pi x f)^2 x C]
EC = 2.5 x DC in