I don't think it is all that likely that Boston could get Siakam without Brown, however, if it was possible, I think it would take multiple teams to get it done.
Perhaps something like this
Boston - Siakam
LAC - Brogdon, Young
LAL - Horford, Porter
Toronto - Morris, Zubac, Prince, Vanderbilt, Christie, Pritchard, BOS 24 1st, GS 24 1st (from BOS), BOS 25 1st swap rights, BOS 26 1st
So the Boston component is acquiring Siakam for Brogdon, Horford, Pritchard, 3 1st's and 1 swap. That trade also does Horford a solid by sending him to a contending team in city a lot of guys like playing in.
Toronto shaves money this year and Zubac is the only guy signed more than a year (at reasonable money so he is a solid asset). They get a look at some guys and pick up 3 1st's (plus if they don't buy them out Morris and Prince could be valuable to use as trade fodder). Solid trade for where they are and Siakam's contract situation.