Trial and error. A foam type filter should let more air into the carb which will make the mixture lean and increase the risk of a seizure. To compensate for this you need to increase the flow of fuel. To do this you increase the size of the jets in the carb and consider allowing more fuel through the needle (midrange). Open the carb and see what jet sizes you have in the main and pilot jets. Buy some bigger ones, it is best if you buy a range but 20 up on the main jet and 2.5 on the pilot would be a good starting point. Also move the needle circlip one notch down the needle (towards the pointy end). Fit the carb back on and attach the filter and start it up. Use a new spark plug. See how it idles and adjust this then see how it rides, taking care and only a very short journey. When riding, and safe to do so, cut the engine using the kill switch and pull in somewhere safe. Take the plug out - bearing in mind it will be hot - and see what the plug looks like for colour. Using the plug colour and how the bike pulls identify if you need to change the jets to a slightly bigger or smaller size. During this you run a higher risk of seizure so slowly does it. The main affects the top end, the needle midrange and the pilot the tickover. That is about it.