The G54 is one of the easiest trans to rebuild.
To rebuild it without a bearing splitter, press, and feeler gauges would end up chipping gears and deforming the end of shafts.
Please do not use a hammer and beat on the gears, shafts, ect

If you want to do it yourself, get a factory service manual from e-bay, and plan on spending time learing how to do it.

Above all else, get it clean inside and out, and take special care to check for mainshaft, and 5th gear endplay.
Toyota sells oversize c-clips to properly complete the job.
Attention to the details will ensure a lasting job.

I spend about 6 hours to go complety thur the G trans.

Marlin