Hi its interesting to read your question and the links that Leics has come up with. the people who have been my family the longest in Morocco, the longest time connections Ive had are from and in Rissani and Ive had quite a number of visits to Rissani and out in the desert around there not that that means I can confirm evidence of Roman presence there from personal experience but historically Sijillimassa was an important saharan city and became more so a southern outpost for Morocco not the Romans being on the ancient caravan routes down into the sahara and subsahara.
There is evidence of Roman presence in many places around North Africa including down through desert areas - there is certainly the Roman towns of Volubilis near Moulay Idriss and Lixus near Larache - Ive also been shown photos of ruins that a local believed to be Roman down the Draa Valley on the way to Zagora which is another major ancient caravan route down into the subsahara regions - so its possible that they were down through the Sijillimassa city - of which there are still a few remaining ruins of walls and a gate - but the books I have that refer to Roman, Vandal, Byzantine, Berber and Arab presence in North Africa talk of Sijillimassa as being in the hands of the Berbers as early as the 1st century and other books I have that refer to the history in the area have no reference to Roman ruins being there still to see.
Ive just also been advised by a man who deals in the jewellery trade who has replied that there is no Roman ruins in Rissani or surrounds. I am yet to hear back from another acquaintance who specialises in ancient Jewish artefacts from southern Morocco who should have a good idea of any Roman history in the area as well.
But certainly a visit to Volubilis is a must do if you are interested in Roman Ruins, and of course Lixus - but many of the grand remaining artefacts and mosaics from Lixus are in the archeology museum in Tetouan which I recommend a visit to in one of my tips in my Tetouan page.
Indeed Morocco is a fascinating country with not only diversity in its present day landscape but its history and peoples.
Circuit of the ksour around Rissani and the remains of Sijilmassa.
Equipment: Allen keys. Adjustable spanner(s) Screwdriver. Hacksaw. Wire citters. Pump. Presta/schrader pump. Easing oil (eg WD40) and lubricting oil. Gaffer tape (aka duct tape, yankees) String.....
Let us say only that the bike I ended up hiring...and I had rejected two others before it had not been lovingly maintained. It was a newish 18 speed basic 'mountain bike, with defunchtgear shift and a slow puncture. As I discoved. The word for pump herabouts is 'bomba'