Like Graham said, transfers are kinda the limiting factor for making good power on a lot of off-the-shelf kits. From the kits I've seen, at least, usually there is enough exhaust port (and they are generally easy enough to increase exhaust duration) to make power, but it's a major job to get the transfers to keep up. I have no idea how well the bolt pattern and deck height and whatnot would match up, but the Peugeot Eurokit you can get at Treatland for $100 has the highest-duration transfers I've seen, 130deg out of the box, meaning you wouldn't really need to touch them.
Is this the kit you're using? https://www.treatland.tv/yamaha-QT50-YT60-malossi-70cc-kit-p/malossi-qt50-kit-318007.htm
If so, it looks like it may have some potential, at least for a lot more than 4.5hp. Post a port map if/when you can.