Well, no, for this really old model of a Bugaboo Frog, there are no compatible spare parts available anymore. You can also try to write to Bugaboo, but really, the only thing you can do with this is to browse classifieds and try to find other Frog strollers for sale, ideally for parts. If you find one that has something broken but your part, the belly bar, is fine, you buy the part or the whole (for more parts later) and swap it for that.
The Fox's belly bar will not work for your Frog, I'm afraid. If anything would be compatible, it would be the bumper bar of the Cameleon series, the older the model, the better, but even there I cannot guarantee you the recent model's bars will fit. That is why I myself would rather try to get the exact one for your model in classifieds or on eBay.