Apart from being present in the NArada PurAna, the mantra is also present in many Agama scriptures. For example, in Ganesha Vimarshini Tantram, Brihat Srikrama Tantram, ViswasAra Tantram etc. In this answer i am quoting the verses from the Viswa SAra.
The Rishi, Chandah etc of the mantra is given in the Viniyoga verses as follows:
Asya Sri MaHaganapati mahamantrasya Ganaka Rishih, Nibrt Gayatri
Chandaha, Mahaganapatir Devata...
For the great mantra of MahAganapati, seer is Ganaka, meter is Nibrit
GAyatri, Deity is MahAganapati...
In Agamas, usually the mantras are always given in a hidden code and never mentioned in a direct manner. In this case, the mantra uddhAra verses (which when decoded gives the actual mantra) are as follows:
Sri saktismaravubhivighnabijAni prathamam likhet| Yenantam ganapatim
paschAdwarAnte varadam padam | uktvA sarvajanam....
First write the Sri beejam (Srim), then Shatki (Hrim), then Smara
(Klim; also known as KAma beejam),then Vighna (Glaum) beejas followed
by the Ganapati (Gam)...
The mantra thus obtained is (without the OM at the beginning) :
shrim hrim klim glaum gam ganapathaye
Vara-varadha sarva janam-mey vasamaanaya swaha
and, which is known as the 28-lettered MahAmantra of Lord Ganesha (AshtAvimsatyakshara Ganapati mahAmantra).
And, for the mantra purascharana the total number of japa required is 4 lakh and 44 thousand.