'Making rafters into trusses' isn't what I meant. You either have rafters or you have trusses.

So, do I understand correctly that the space on the left in the sketch is the garage?

I suggest you do most of your planning with your parents' potential needs in mind. I'm not suggesting you put in ramps and such now, but wide doorways to the Master Suite are easy now- a challenge later.

Are you sure that moving this house is economically practical? Might be better in the long run for your parents to build something to their needs and taste than try to get by with some well-intentioned but half-measure adaptations.

You could also 'capture' the garage space in the move and build a new garage. Probably more space than you need but you could do a lot with that.