If you read the engine requirements in the manual, it says anything from a 40 to 91 will work.
I have had a Goldberg cub for quite a few years - initially powered with an OS 70, but it seemed a bit underpowered when flown on floats.
Replaced the OS70 with a Saito 91 purchased on ebay several years ago. With the Saito 91 it comes off water with authority. Flies wonderfully at half throttle, floats or wheels but will do some pretty spiffy stuff if you open up the throttle.
You need to be careful not to go full throttle when taking off or it will ground loop from the torque.
Recently crashed it and rebuilt it using ARF colors - or close to thereabouts.
Pics included........