Rainforest plants would do OK. Bugs would thrive!
Long before there were plants on land, there were microbial mats. The heyday of marine microbial mats was well over a billion years ago, with the evolution of multicellular grazers and tunnelers pushing them from dominance.
On the land the microbial mats of cyanobacteria and other bacteria did well. There are also fossil traces of these from over a billion years ago. Microbial mats of this sort still do well if they get a head start and grazers are excluded.
Life on land in the Proterozoic: Evidence from the Torridonian rocks of northwest Scotland
The Stoer Group and Diabaig Formation of the Torridonian succession in
northwest Scotland are late Mesoproterozoic to early Neoproterozoic
(ca. 1200 1000 Ma). Features preserved on the top surfaces of fine- to
medium-grained sandstone beds in a number of stratigraphically and
geographically separated localities are attributable to microbially
induced sedimentary structures; these include wrinkle structures,
remnants of apparent microbial crusts, and indications of original
cohesiveness and pliancy in sand-sized sediment. The surfaces on which
the microbial structures formed were exposed subaerially (abundant,
deep desiccation cracks and locally pedogenic structures) in alluvial,
interfluve, and lacustrine margin settings, and many of the structures
developed in areas well away from the perennially wetted regions
adjacent to shorelines and fluvial channels. Thus, these features
indicate that Earth's biosphere had adapted to and colonized land
surfaces many hundreds of millions of years before the dawn of the
There is evidence of ancient soil and even multicellular life of some kind dating from the early - middle Cambrian, also before the date proposed for the forest transplant.
The Great Oxygenation event had reached its final and current stage by 850 million years ago, also before the described transplant. The atmosphere would be close enough to our modern atmosphere for plants.
Hundreds of millions of years of terrestrial microbial mats is plenty of time to make good soil, especially in someplace like an alluvial plain that concentrates the rock breakdown products of these mats. Good soil, good atmosphere, water and light means the forest would thrive.
But what would really thrive are the microbe grazers and their predators. Springtails, slugs and nematodes would quickly spread over the delicious microbe-encrusted earth. These little grazers would move out from the forest and feast on the defenseless microbial mats. Predators that eat these creatures would also have a heyday.
There would be a nice island of forest. The rest of the world would be overrun with bugs until the mats were gone and population crashed.