Whether or not the chassis freewheels is not a big issue in my view, it's how smoothly it runs under power. Fit the motor and apply power direct to it and see how it all turns. Maybe try it first without the rods on to check that the gear meshing is OK. As the saying goes, you can't test a system at rest. A bit of play is not a disaster. I have an 0-4-4T which is around 40 years old, has two loose crank pins and a fair bit of wear in the 20thou p/b frames (no bearing in those days!) and you can turn the non-driven set of wheels about 1/8th of a turn, but it still runs sweetly.
To try it on the track, however, you will need to add some weight, something like a couple of chunks of lead stuck on top fore and aft somewhere with Blu-tack, ideally so that the weight is evenly distributed. A chassis on its own is far too light for effectively collecting current.
Jim