They will use the attitude control engines instead of the main engine.
JAXA decided to carry out orbit control of the AKATSUKI using its liquid-fuel thrusters for altitude control (or the reaction control subsystem, RCS.) Based on this decision, almost all of the unnecessary oxidizer was discarded in Oct. 2011. As a result, the satellite became lighter, and remaining fuel can be more efficiently used for orbit control.
Akatsuki's orbit seems to have been a fortunate accident. At least, I could not find a statement that this was planned beforehand.
On its way to Venus, Akatsuki was in a heliocentric orbit that intersects Venus'orbit. And with limited delta-V available for insertion, this orbit must have been reasonably similar to Venus' own orbit.
When orbital insertion failed, it was inevitable that Akatsuki would cross Venus' orbit again. JAXA calculated it would be possible to aim Akatsuki to meet up with Venus again in 2015. To achieve this, Akatsuki performed several orbital maneuvers in 2011 with a delta-V of 243.8 m/s that would lead to it meeting Venus in 2015.
By december 2015, Akatsuki will have made 9 orbits around the Sun, while Venus has made 8 orbits in the same time. JAXA got lucky in the sense that Akatsuki's orbit was close enough to an 8:9 resonance that they could achieve 8:9 exactly with the limited delta-V available to them.