Generally the engine grounds to the frame. Take any engine mount apart. Clean off any paint or powder coat or rust where the parts touch metal to metal. A bit of grease will help keep things clean. This will give a good ground from engine to frame. Now do the same to the battery ground. Clean metal to metal contact.
Any other grounds the hook to the frame need good clean metal to metal contact.
This will cure at least 75% of your electrical issues.
Leo