A 120mm mortar might be workable with some jury-rigging. For instance:
The CWS experimented with mortars mounted on landing craft, including the LCIs (Landing Craft Infantry) and LCTs (Landing Craft Tank). It took the view that mortars could support an amphibious assault in the crucial period of an invasion, after the naval and air bombardment let up so that troops could land. Mortars could not be placed directly on the bottom of landing craft since there was no way to keep the recoil from kicking mortars backward when the piece was fired. In addition, the terrific pounding might damage the bottom of the vessel. Technicians rigged an oblong wooden frame, filled with a mixture of sand and sawdust, on the floor of the craft. A thick slab of wood (several sheets of plywood) grooved to take the bottom Y-spade of the baseplate, sat on top of the sand-sawdust filling. The Amphibious Training Command, Camp Carabelle, Florida, to which CWS sent the firing platform, saw the utility of the device and asked the service to design a standard model. [Vol. II, pp. 136-7]
This passage talks about mounting 4.2 inch mortars on Higgins Boats. Such a rig could be used to mount a 120mm mortar on the deck. There should be lots of bracing below decks as she's an ice-breaker and will have lots of reinforcements.
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