Chain problems with eng swap..???

I swapped the enging from one of my bikes, now I have a problem with the chain "walking" off the rear sprocket to the 'inside" of the sprocket between spokes and sprocket. I set the chain on the front sprocket and the rear sprocket to "eye" up the "run" . The small front sprocket is further "in" and causes a "diagonal run" which throws the chain off.... how can I remedy this?... I tried to remove the front sprocket thinking I could "shim" it out... never did get it off... then I thought I'd remove one of the washers off the rear wheel but after consideration I realized that would not change the "relationship" of rear to front sprocket... anyone have any solutions? who has experienced similar issues?... remedy?..... thanks
Bob

Thanks for the replies and input. Misteright I did look at that outer rag joint, and thought about washer tinkering on the wheel... thinking the rag joint is "holding the sprocket out of align".. but wouldn't have a clue what to use for a thinner replacement. Norman thanks for the suggestions and info on the offset of mount... I'll try again...