Hi,

You will probably get problem with center of gravity (CG)the OS 120 4-stroke that weight about 35oz /1000 gram. Daddy Rabit was designed for an engine about half of that weight. I suppose You can compensate some with placing standard servos for rudder and elevator in the tail near the control surfaces and maybe a larger then neccessary battery back in the fuselage. But it is always better to have the mass gathered close to CG and not spread out in various directions.
Another problem You will have is the larger prop diameter you need with OS 120 4-stroke compared to a .60/.61 2-stroke - prop clearance to the ground if you make it original trigear will be a problem but You could make main gear/taildragger and that will solve the problem OK.
You will also have it overpowered in my opinion but it will probably fly OK anyway.
It will probably work but it will not be good really. There are so many good 2-stroke, 4-stroke and electric motors out there that will fit a Daddy Rabbit better and make the planefly as intended.
Daddy Rabbitwas enhanced in many versions during the years Jim Whitley flew it both at top US level (NATS etc) and at World Champoinships like in 1969 (10th place),1971 (6th place) and 1973 (8th place). As far as I know Daddy Rabit (I) was in 1966 and then up to version X that was the last version (I do not know the exact year but probably 1973 since at 1973 WC he had verion X).
In 1966 version of Daddy Rabbithe had a Veco (Lee Custom) .61 according to the article in Model Airplane News May 1967.
In 1971 WC he had Daddy Rabbit VII with wingspan 1638 mm (64.5 inch), weight 3714 gram (8.2 lb), Webra 61 (Black Head) with Silence Air silencer, Top Flite 11 x 7 3/4prop and Pro Line radio.
A Veco 61 from that era should not be impossible to find but may take a while.
A Webra 61 Black Head is not that hard to find today even new in box if you have some patience (just check the big auction site and sooner or later it will be one there).
On the other hand a modern engine like OS 55AX or OS 65AX will work fine also.

Weight: In the MAN May 1967 article it is mentioned 8 pounds dry weight and 9 pound take off weight with fuel.

Good luck and please make a build/assembly thread here regardless what engine You decide to use.

/Bo