You can get the Model M style US International layout from Unicomp; they would know the specifics.  Please order a new Model F keyboard with the “bring your own keys” option and then you can install the Unicomp set yourself.  However I do recommend the brand new XT quality Model F keys being made from new molds – you can program in the US International functionality into any key set using the xwhatsit GUI configuration software.  Also some operating systems may allow easy changing of the layouts without having to change the configuration on the keyboard itself.