To be accurate, it would be easier to start from scratch. The 1st gen body is narrower, and the front and rear overhans differ greatly. The interior is also completely different. The only thing that is shared between the 1st gen., and the Scout II is the wheel base. There were several diecast Scouts produced, non of which were very accurate.
The 4 cylinder engine was available in the Scout II's thru 1980 although it was a 196 ci, in place of the 152 ci in the earlier Scouts.
I have started on several occasions to master a 1st gen body, but, I hate working on them at work, so, its very hard to get enthused about building a model of one!