Motor switch

The 186 would run well on PULP, providing you use an additive to make up for the lack of lead. The octane rating of PULP will suit the 186, but the lack of lead will accelerate valve seat wear. In terms of switching to a Commodore motor, there's no gain unless you go for a Nissan engine, as used in the VL, or a later V6. Earlier Commodore motors ran on leaded fuel just like your 186, and I'm guessing the modifications needed to fit a Nissan or V6.