I take it that the existing transformer has a primary voltage of 220V AC at 50Hz from your first post. If that is the case, then your sine wave oscillator driving it has to be able to supply that voltage for it to work when presented with the primary impedance on its output, which I take it is what you are talking about when you mention current boosting. To increase the current drive is not trivial, you would need high voltage FETs or transistors in a current boosting configuration. It is common to put the primary in the feedback loop of the amplifier as well, to get a high impedance current source drive.
If driving it from a current boosted high voltage source is going to be difficult, and it will take careful design, then there is another option.
Lower the oscillator voltage, say by a factor of 10, and use a 1:10 step up to get your 220V AC drive voltage. You will have a corresponding increase in the current drive necessary, plus loses, but you are not working at dangerous volatage levels as you are in the first option.
In either case, another option may be to go to a PWM scheme, to keep power dissipation to a minimum.
From your schematic, it appears that the secondary power requirements are quite low, is that correct?and that the major current component is primary magnetising.
Without a lot more details, oscillator properties, as well as details of the transformers primary impedance, and power requirements, it's hard to say what it is you need precisely.
Cheers,
Richard
