I would rebalance it.

The whole point of balancing it is to match the counterweights on the crank with the weight of the rods & pistons so that the engine rotates smoothly without vibration. If you have no idea what pistons or rods were in it you have no idea if the balance would be correct with the new pistons & rods.

If you don't rebalance it you may have a nasty vibration that won't go away without a complete teardown and balance. The cost of balancing now it is cheaper than the cost of a teardown.