I found Athearn Blue Box diesels pretty easy to convert. I have a set of F-7A and F-7B locos I've converted. I've also converted a GP-38-2 and and SW-1500 (actually an SW1200). Isolating the motors was a simple matter of bending the little Phosphor Bronze tab that makes contact with the frame on the underside, so that it no longer made contact and putting electrical tape where the tab used to make contact on the frame, to protect against possible contact, then soldering the Orange and Gray wires of the decoder to the Phosphor Bronze clips that keep the motor ends together. The lights where generally easy enough as there seemed to be space enough to get the job done.
I have found the Life Like Proto 2000 locos to be a bit more complicated as, space for installing the lights and decoders in these locos is at a premium!