It's really easy to build, it sounds as though you're about as good at that sort of thing as I am and it really put me off to start with as I really am hopeless at anything like that and was pleasantly suprised at how easy it was taking into account my complete lack of ability !!!!!!
The only real bit of advice are take your time over it and just check things before you glue them, if you get a motor that is too big by chance again I can talk you through what to do and it's not hard, just needs ideally a dremmel or something similar although I didn't even have that at the time and just used a craft knife with a new blade to trim some of the bits back.
I'll post on another site as well and see if I can find someone who's got a motor that slots straight in.