Besides having to clear the existing gearbox, the question becomes whether the Atlas movement and gearing is suitable and accurate enough for a turntable nearly twice the size. Also whether the appearance suits you.
The Atlas turntable uses a Geneva movement which pauses and is indexed every 15 degrees (older versions used 30 degrees). Larger turntable are generally indexed at 10 degree increments or less, and are set up not to stop at every possible track location.
If you want to turn an engine 180 degrees, do you really want the turntable to pause 11 times as it rotates? If you speed up the turntable too much to compensate, your little engine crews are going to be getting whiplash. The extra stops may be able to be eliminated by sealing the slots on the Geneva wheel, but this will require some disassembly (note I have not performed this surgery myself).
Do you really want the fewer and more spread apart roundhouse stalls that result from the 15 degree indexing? The Walters turntables and roundhouses are set up for much closer spacing of stalls.
Finally, will the indexing be accurate enough with a bridge nearly twice as long?
Although the Atlas turntable is cheap, it has a lot of limitations, too. Most of the modification stories that I have seen have not lengthened the turntable by much to avoid even more rebuilding. With Walters offering highly recommended, reliable, much more realistic turntables in both 90ft and 130ft sizes (the latter has an 18" bridge, and should accommodate just about any steamer), trying to modify the Atlas turntable into something it is not becomes a case of just not worth the effort.
OTOH, if you are looking for a base to make a 70ft or smaller TOC turntable, the Atlas is a good starting point, as numerous articles have shown.
just my thoughts, your choices