The c90 is a suprisingly good touring machine, although I would suggest you stick with the cub 12v cdi variety.
As for mods, firstly go light. and I mean it.
Get a granny basket on the front, this will house a 1 gallon plaggy easily, and apart from giving you a total range of 200+ miles, will get some much needed ballast over the front wheel.
what time of year? and expected weather conditions? the leg guards can be extended giving fantastic weather protection, at the expense of drag.
What terrain? if the bike will be used mainly on flatish roads, up the gearing 1t bigger on the front, less screaming, but it will accentuate the chasm between 1st and 2nd.
get a single seat conversion, mine was £40 off ebay, saves taking all the luggage off to re-fill. the seat was a bit weak so I reinforced the mounting points.
For serious winter use, I used an old clutch cable setup, to put the rear brake on the bars, so on ice, could use both feet and slow down at the same time.
The headlamp is a bit feeble, some people use an led ear bulb and swap the 25w glow-worm for a 35w halogen bulb.
I carry all the heavy stuff (lock, tools etc) in a small bag, hanging from eyebolts that replaced the upper leg guard mounting bolts (m6).
This gets the weight forward and down. Don't overload the top-box, this will make the steering seriously dodgy in sidewinds, and when being overtaken by truck. (a natural hazrad to c90 riders)
As for spares, the front and rear use the same tyres and tubes (bonus), so a couple of tubes, spare plug, lead, cdi,reg/rec, split link for the chain etc.
put a new good quality chain on, soak it in ep90 gear oil, and every day, open up the chain case and lube it. it will stretch a bit but will last forever.
Take the wheels off, stick new bearings in (dead easy), remove the brake pivot points, clean and lube with moly grease. people alway neglect this, the brakes become stiff and less responsive. The same people always critisise the drum brakes, when fettled, they will lock the tyres, what more do you need.
The c90 is a fantastic machine for the newby, incredibly reliable, crash proof (oh yes) cheap to run and maintain, field serviceable etc. They are not funny on motorways, so stick to the small roads and you can fairly tram on. With the comfy riding position and fairing ,my record is 450 miles in one day, rotterdam to bavaria in winter.
the weak spot is the exhaust valve can burn out, this is due to the small .05mm clearance, and lack of maintanance and it is worth checking every oil change. very easy, just unscrew the inspection port. and keep an eye on the oil level, they tend to burn a bit when past 12,000 miles.
They are a great giggle, people always come up to me and heap praise on them, most bikers have had one at some stage, and have a grudging respect for them!