i know john bitmead ( aka darkhorse ) . made a aircooled engine with a turbo etc using metro bits and gubbins .
i think it lasted , he sold it awhile ago though .
i suppose anythings possible , but i wouldnt bother modding a aircooled 126 eng, might aswell swop it out for something altogether bigger?
as darrell said, anything can be acheived .. i suspect youll need deep posckets though, and just remember to tell the insurance comp of your mods ..