SB-401 = no AM at all, it was never designed for that. With major modifications it might be possible, but the stock design doesn't come close.
SB-301 = can receive AM but wasn't really designed for it either.
Easier to buy a cheap old AM rig from the 50s and just go with it. All the new DSP rigs (Japanese or American) work well on AM, as the modulation is all done by DSP so the difference between modes is just lines of code. However, the typical 100W rig will only run 25W of AM power. The old plate modulated boat anchors of the 50s run "full power" on AM, so even an old Ranger, rated 65W of power, will do that on both CW and AM equally well. There is a difference.
But anything that gets you on can be fun! At any power level. The "big guns" mostly have HUGE antenna systems and will hear you almost regardless what you use. The real work is done by the antennas.
A government which robs Peter to pay Paul can always depend on the support of Paul.
-- George Bernard Shaw