Hi M&M and re- to the forum
there's not too many differences on the engine specs for the heritage, in fact I have the official yamaha SH manual I use for my 2A2 model.
1981 is the SH model and is 4R4
http://www.cmsnl.com/yamaha-xs400sh-1981_model8826/partslist/
not surwe what parts your after, but you may struggle for seats and badges.