garage reinforcement

Hi,

My friends garage is 23 feet long X 16' wide X 16.f' from the slab to the peak of the roof. The south and north walls have a double top plate and the east and west walls are built entirely out of cinderblock. He has some substandard collar ties installed now. Two spans of 2X6 run north south, not spanning the whole distance and are nailed together in the center and otherwise unsupported. there are some upper 2X4 rafter ties in the upper 3rd of the roof space but not on every rafter (rafters on 16 oc. The roof slopes to the south and north along the 23 foot span of the garage, and the slope is very slight (3/12 or less) . It will hold loads in the winter from snow and ice. I want to install collar ties that sit on the top plates of the north and south walls but I'm not sure what to use for that 23 foot span. East/west is a shorter distance but there's nothing there but the stone wall, and spaning them this way wouldn't keep the rafters from pushing out on the garage walls (garage is completely detached from house). I was thinking of putting in a central beam (lvl, runnning east to west across the 16 foot span) and sitting it on joist hangers anchored into the cinderblock, then I would tie into the lvl beam with 2x10 on joist hangers and sitting on the top plates of north and south walls. He wants to store some light stuff up there so we would clad the collar ties with plywood and be done with it. Will this solution keep the north and south walls from pushing out under the weight of the roof, or is this overkill and am i missing an easier solution. thanks for your time.
Neil