We should have your MODEL. The year is unimportant for servicing, but precise model/serial is critical. I'm assuming you are working on an NSF18B2, right?
Don't put a torch to the housing (as was suggested) to expand the bearing and housing. The uneven heat can crack or warp the LU housing, and can carbonize the LU lube in the forward ball bearing, aside from the obvious burning-off of the paint, etc. The boiling water trick might work, and a warm electric-fired oven could also expand the housing a bit for you, but those techniques are pretty cumbersome.
There is a special/expensive internal gear/bearing puller that will work. Some old-time OB shops use them. You could likely buy or rent one from your local auto parts or tool rental house. But they are expensive, and require a little expertise to use. FWIW, The gear is not retained by any clips or pins; It's just floating in a very-close-fitting bearing. The bearing likewise is a very-close-fit to the LU housing.
There are 2 regularly-used methods to extract the A gear.
The first method is strictly mechanical: Pull it out, using a pliers-type tool that will fit between the housing and the rim of the gear. Most shade-tree shops don't have such a tool. Or, a few stiff whacks of the housing on a soft wood bench surface (prop end down) will usually get the gear out. Sometimes the bearing comes out with it. This works over 90% of the time, as long as there isn't any rust on the gear's bearing journal.
The second way is hydraulic, and was used all the time to pull clutch pilot bushings from the crankshafts of car motors. Make sure the shift rod and bushing are still fastened in the LU, to contain the hydraulic pressure you are going to create. Inject regular chassis grease into the cavity in the center of the gear until it is completely full. Use a "needle greaser" adapter for your grease gun, and try not to get too many air bubbles in the grease load, as air compresses easily, and you need non-compressible medium in the cavity. Then slide the close-fitting prop shaft and push rod into the gear part way. It should bottom in the grease before it is seated in the bore. Then whack the end of the shaft (rapidly) with a mallet (not a steel hammer that might damage the threads on the shaft), and the hydraulic pressure will pop the gear out.
All that said, be sure that you also inspect the B (pinion) and C (reverse) gears, as well as the lugs of the clutch dog, very closely. The pinion takes the brunt of the wear, but any damage to one gear can/will hurt the faces of the teeth of the other 2 gears as well. You may need more than just the A gear. And when you reassemble everything, don't forget to set the lash (by using the appropriate shims), or all your work will be for naught. If the lash is not correct, the gears will self-destruct. Full information on the lash is in the Factory service manual, available from any dealer (including me). If you are using an aftermarket manual, consider using it for a more appropriate purpose, such as a doorstop, or woodstove tinder.
