It would help to know what fork is currently on the bike. G2 just means it has a 51mm offset, which most modern suspension forks have. If you want to switch to a rigid fork, you should try to match the (sagged) axle to crown height of your current fork and the rake. To get the sagged axle to crown height, you would normally subtract about 20 to 25% of the fork travel from the unsagged height. For example, if your axle to crown height is 500mm and the fork has 100mm of travel the sagged axle to crown will be in the 475 to 480mm range.

The Surly Ogre fork only has a 43mm offset. The Surly ECR is a bit closer at 47mm. They appear to have several different 29er forks with differing axle to crown heights and offsets, so you may be able to find something close.