The main drive shaft oil seal can be replaced with the transaxle installed or removed. Remove the engine, clutch release bearing, and the seal from the transaxle case.
Before installing the new seal, lightly coat the outer edge of the seal with sealing compound, and lubricate the main driveshaft and the lip of the seal. Then, slide the oil seal onto the main driveshaft, and drive it into position with a suitable sleeve or pipe. The seal should be slid onto the shaft very carefully in order to avoid mis-positioning the spring around the lip.