This is my helicopter, and if you have a Jetcopter, you will recognize the Jetcat parts, but the rest will not be the same as yours. To start with, remove the swashplate and everything above it on the mast.
Next, you need to remove the top bearing housing, the bottom bearing housing and the loosen the 3 collars on the mainshaft. This will loosen the mainshaft and allow you to remove the locating pin in the autorotation hub. Its just below the hub and the next picture shows where it is.
Now you can remove the mainshaft completely, and then if you remove the screws in the support bearing holder, thats the one which bears on the edge of the main gear, you will be able to move it clear of the main gear and remove the main gear. Later versions of the assembly have two of these support bearings and both need to come out. You can see this one just above the main drive pinion.
Later versions of the assembly have three holes in the mainshaft, but mine is the early one. The new mainshaft needed is available from Jetcat USA or you can drill another hole in the main shaft yourself as it is not hardened. The new hole needs to be 40mm above the old one.
Re assembly is straightforward, don't tighten anything down until it says so. The support bearing holder or holders need to be slid into position, but well out of the way of the maingear. Slide the maingear back into position and slide the mainshaft through it. Fit the bottom clamp back on, collar upwards, and then fit the bottom bearing holder in place, bearing upwards. Slide the double collared clamp down onto the mainshaft, slide the top clamp on, collar upwards and then slide the top bearing holder on, bearing downwards. Slide the pin through the hole in the mainshaft. The picture shows the order the parts should be on the mainshaft.
Now to set it all up. Tighten the bolts into the top bearing holder. Use loctite on all of these bolts. Push the bottom clamp up against the auto hub as far as it will go and ensure the pin is in its groove in the top of the hub. Tighten the bottom clamp in position. Slide the middle clamp down to the pin and tighten that in position. Now ease the bottom bearing holder up with the screws in their slots, until the maingear is in position with the drive pinions. It needs to be pushed up firmly, but the maingear must not bind and must not have any play. Fix the bottom bearing holder in place. It is the position of the bottom bearing holder which removes any up and down play in the maingear. Push the top clamp up to the top bearing holder and tighten it into place. Slide the support bearing holders into position and fit their bolts. Slide them up so they are just clear of the maingear. They should not be touching the maingear.
Check everything spins freely with no binding or play and snug all the bolts up. Now you need to rebuild the washout, flybar and head for reversed rotation. The tail rotor will not change its direction.