Cool, busilding a Mills .13 engine would be great fun. Congratulations on your success.

A 2.5 diesel engine ought to do the trick for you. Maybe the MP-Jet 1.8cc Atom or the MP-Jet 2.5cc Letmo would work too, hard to say without trying one though.
Carlson's website is basically for reference only. You need to call him on the phone instead to see what he has. Carlson's web developer left him high and dry and he doesn't have the password to the website to edit it anymore. So he hasn't been able to updat the website for several years now.

Pe Reivers still has a couple of the MVVS 2.5cc diesel engines for sale though.
http://www.prme.nl/