I've been running a MG in the 62 for about 3 years w/ stock wiring and have had no issues. Others, specifically Landpimp, have upgraded the wiring from the MG to the battery and have added a mega fuse as a replacment for the stock fusible link set-up to protect the battery from over charging. I think that wiring upgrades really depend on why you're adding the MG. If you're looking for more power for winching and supporting a second battery then upgrade the wiring it's not that expensive. If you're just looking for an alternator to replace an OEM and you're not placing additional load on the charging systmne then the stock wiring is probably fine. Use search and look for posts from Landpimp on Mean Green - he has some great photos and descriptions on how he upgraded the wiring on his 60 to support his dual battery set-up.
Hope this helps.
Harry