like the previous post states you sure can. but to make it work you need to do a lot of work. like change the gauge cluster re-wire or alter your wiring harness to accomodate the turbo APC and so on. there are a lot of post about this. do a search on engine swap or turbo engine in 900s. you should find all kinds of good stuff.
I have a 900s and even asked my mechanic about that one. he said to just buy and 900 turbo and drive that. But I know what you mean. I love my 900s and dont want to part so I will either do a NA upgrade to get a little more HP out
I have a 1990 900T convertible that I lovingly fixed up. Pulled the engine, all new seals/gaskets, and lots of little bits off of ebay. Some f-er in an Equinox pulled out in front of me and the passenger side front end is toast. Found a 1992 900S convertible with tranny problems. It's an automatic so I guess I couls just swap trannies but would like to give it a little extra "boost" if I could.