Dean,
The BSE line stands for Bar Stock Engine. Those are the machined case versions. Performance wise, the BSE-90L runs a tiny bit better than the SJ-90L cast case version.... but either will work.
The 90L is more than you will ever need in the Little Toni. If you go with the 120, you have do some serious prop experiments, and probably use at least a 13" diameter prop .... that will create enough drag extra to minimize any speed gain.
If you look in the jett web site's hall of fame section, there is an AT-6 racer with a big block 120 installed. There is a good deal of information available with that photo about the application. It works well in the AT-6 turing the larger diameter prop out in front of the big cowl.
The 90L is also the same size as an OS61 or 91FX, which means it will drop in without any serious hassles (does require a special muffler extention kit we have available).
The weight is not really an issue. The BSE-120L weights about the same as the 90L. The big block 120 is about 3 oz heavier.
Down the road, the 90L will be very happy turning a bigger 13-14" prop with a simple muffler change. But in the short term, its your best Jett engine choice for this application would be the 90L.
(edit - link to the above mentioned AT-6 article
http://www.jettengineering.com/hall/at6com.html )