There are several ways, use a normal audio amplifier driving a mains tranfromer, use a ferroresonant transformer driven from a normal h-bridge, or a class-d amplifier driving a high frequency transformer - explained below.
I would use a ferrite core transformer connected to a full h-bridge connected to a PWM working at about 40kHz and put a low pass filter on the output to only allow the 50Hz or 60Hz signal to pass.
A pure sinewave inverter isn't that complected, all it is is a class-D amplifier working at an extremely low frequency; it's a lot easier to build than a class-D audio amplifier.
Also not only will it give a good sinewave, it'll be more efficient and more compact than a modified sinewave inverter.
Google for class amplifier if you want more information.