Hi R.Peck
I am afraid to say that you will NOT find a "CAT" engine to fit into your "Allis Chalmers" model "G" tractor. The smallest diesel engine that "CAT" built was the "D3400", or the "D311" both of which powered their then "D2" tractors. This would be far to gigantic to fit to a model "G" - totally inpracticable owing to the physical size and the high weight of those engines. Alone one of those engines would weigh much more than the complete model "G".
Regards
Alan T. Lewis
Christchurch.
NEW ZEALAND
If you are talking about the Surplus center in Nebraska they some time have Perkins engines,and they may advertise them as Cat engines as Cat has bought the Perkins company in the last few years.Hope this helps,and you find something that works for you.Ray