Dear mupchu,...
You asked:
ORIGINAL: mupchu
I am assuming that if I wanted to build the 120 a good engine choice would be an os 1.20 4 stroke etc? or what engine range would be good for each?
Yes,... I've seen (or heard of) a Boxer 120 with the O.S. 120 Surpass. Just keep in mind, these planes were designed with the fuel tank located on the CG; therefore a pump is required to supply fuel to the engine. In such case, I'm assuming you are talking about the pumped version of the 120 Surpass.
From the specs listed above, the Boxer 120 is NOT a two meter bird; therefore (if built light) a 120 Surpass should provide adequate power. However I assume most Boxer 120's are being flown with a YS 120.
As far as the other sizes are concerned,...
Recommended engines for the Boxer 60 are YS61RE or OS 61RE, however the Boxer 60 can also be fitted with a non-pumped or non-piped 60-70 two cycle engine, (relocation of the fuel tank would be required).
The Boxer 90 was designed with the intention of utilizing the (then new) Saito 91 (with the Saito 120's pump system), or the YS 91. The intent was to use the "stock" mufflers that come with these engines resulting in a substantial cost savings.
Hope this answers your question.
Sincerely,
Steve T