First thing to do is to check the actual secondary voltage using a 220V to 100V step down transformer so as you have a record. The hard part on this process is separating the E-I core of the power transformer. You'll have to submerge the transformer on a gallon size container just to cover it for a day. The insulation should loosen up by that time and carefully dissassemble, bracket first and next the I core, followed by the E core. Use appropriate tools to do it. What we are converting here is the primary winding. Take note that this is rated at 100V, so after getting the number of turns of the winding, get the computed turns ratio and calculate the required turns for 220V. Now the wire gauge to be used would be almost half the diameter of the original winding.