The Business Empire of Sun Wukong, the Great Sage Equal to Heaven

How the transmigration happens:

Aftereffects of the pilgrimage: After completing the pilgrimage, Wukong was named the Victorious Fighting Buddha. But life in the Heavenly Court was dull, and its rules were strict. An accident, such as a surge of spiritual energy in the Peach Garden, a space-time rift, or an experimental magical artifact going out of control, swept him into a temporal maelstrom and sent him crashing into a modern city.

Conspiracy behind the Journey to the West: The pilgrimage was a setup all along. After attaining Buddhahood, Wukong gradually uncovered the truth: the Buddha and the Heavenly Court had ulterior motives. When he tried to resist or investigate, he was ambushed and cast into a space-time rift.

A modern-day summoning: A wealthy person, scientist, or mysterious organization conducts a forbidden experiment, perhaps to summon the God of Wealth, and accidentally summons Sun Wukong: the strongest fighter around, but penniless at the time.