How can I regulate the output voltage in this inverter circuit
Use a buck regulator to control the transformer centre-tap voltage. I've used this method and it works. 

Advice: you will probably need snubbers across each half winding of the primary in addition to what you have shown in your schematic. This prevents excessive back emf destroying the MOSFETs. I would also consider an output LC filter to get rid of a lot of the output harmonics that could cause RFI to other equipment locally.