If it is like most Mercs, you need to pull the gearcase and powerhead. You will need to work around any corrosion that inhibits your removal.
At this point you may be able to remove the lower motor mount covers and cages, Now pull out on the midsection and she may be free. she will need to drop straight down to clear the water tube. Tilting the motor may help.
If not, try the following steps.
Remove the shift rod.
Separate the exhaust extension halves(top and bottom). Look for some bolts under the powerhead, when it has been removed. You may or may not have any. However, these will likely break.
Now the swivel steering rod and upper motor mounts can be removed, pulling them up and out of the swivel rod yoke. There may be a split ring under the lower swivel rod yoke. This would need to be removed.
The midsection should be free, although some models have more bolts from the exhaust extension to the midsection.
The motor mounts usually rust solid to the mounting bolts. You can reuse them is they are good. Source them before deciding to replace them, as they might be hard to get. The motor mount bolts are likely grade 8 steel.
You will need a powerhead gasket and exhaust extension gasket, plus a midsection gasket.
Good luck.