And David is quite right that the Digital architectures are really extinct.
[Now I wonder if Compaq would open source the IP of Alpha. I think they licensed a lot of it to Intel so sadly they probably still make a bit of money from patents there.]
All of the ISA specifications are freely available[1] and you can buy a license to manufacture SPARC hardware for $99.
also though any us patent filed since 01995 expires 20 years after filing, so any us αxp patents filed in the period 01995–02001 have expired already, and us patents filed before 01995 expire 17 years after issuance, so we only have to worry about pre-01995 us patents if first dec and then compaq and intel managed to delay their issuance, lemelson-style, until 02005 or later, which seems extremely unlikely
so αxp is almost certainly already open-source as an isa
dec's implementations are i suppose not open-source though, and casual glancing around on opencores and the like doesn't turn up any obvious candidates