Morgi is already accepted as a translation from English to Welsh. Siarc is the more common term
I take it siarc is a loanword from shark, but would 'Morgi' be the more indigenous word for shark?
It doesn't look like it according to http://geiriadur.ac.uk/gpc/gpc.html
'Siarc' goes back to 1725, while 'Morgi' is more commonly used for the 'Dogfish' family of sharks and goes back to a similar time according to the 'geiriadur'.
Y termiadur, which defines all the terms used in Welsh medium education also differentiates between the two words:- http://www.termiaduraddysg.org/?lang=en