A quick look at Collins SCOTTISH CLAN & FAMILY ENCYCLOPEDIA by George Way of Plean and Romilly Squire (now of Rubislaw) shows no recognized chief of the Ewings.

However, even if there was a chief of the Ewings, it would not be strictly necessary to obtain permission to display the crest on your sporran. What is not generally known is that, unlike arms, crests are not unique to a single individual. In addition to Ewing, the crest of a demi-lion rampant holding in its dexter paw a star is also used by twenty other families. So, as a strictly "historical" item there is nothing to prevent you from wearing the sporran.

However, since a crest (in and of itself) can not exist without a coat of arms, you may not "adopt it" as your crest unless you first receive a substantive grant of arms. You may do this by applying for a grant of arms from one of the following heraldic authorities: the Bureau of Heraldry in the Republic of South Africa; the Chief Herald of Canada; the Court of the Lord Lyon; the Chief Herald of Ireland; or (in exceptional circumstances) the College of Arms in London.

As part of that grant you may request as your crest the demi-lion on your sporran, as used by Alvares, Craig, Dillon, Elstob, Eurn, Frew, Gladhill, Henlock, Holburne, Hulburn, Inglis, Lamb, Lambe, Liddell, Lidell, Liddiard, Roberts, Stable, Starr, St. Lyz, and yes, Ewing.