Yes that's a Petter BA engine for sure. Your best bet is to keep an eye out for ebay listings, as these manuals appear on there quite often.
Good engines when they are looked after, but they are not tolerant of abuse. Depending on who you talk to they were widely thought to not be the best engine Petters made. Many advanced features, including metric thread forms on all the fasteners.
Early versions were prone to head to cylinder barrel leakage (no head gasket). Petters redesigned this aspect but it still didn't totally cure this problem.