How much and what assistance work will depend on your current exercise volume, closeness to contest, and weaknesses.
With that said, here are some ideas.
Squat: Good mornings, lunges, squat with bands, box squats, paused squats, front squats, high bar squats, ultra wide squats, bulgarian squats
Bench: Board press, floor press, military press, close grip bench press, incline press, bench press with bands,
skull crushers (barbell or dumbell
Deadlift: Romanian deadlifts, deadlifts with 25 pound plates, deadlift w/ bands, hyperextensions, rack lockouts
Abs: Ab wheel, weighted incline situp, kneeling cable abs, russian twists, leg raises
There are so many to choose from.