I have fished aluminum boats for many years. I have tried to re-rivet them but it is usally just a temperary fix due to the stress put on these types of boats...expecially older models.
Your best option is to take it to a local welding shop and have them weld the braces and rivets. You can also fix any small holes that may be in.
Most places will charge you by the job, not by the hour. its not that expensive (last time I had heliarc repairs it run $80.00) which is not at all expensive if you factor in "your" time that it would take to drill, re-rivet and then seal the holes (which would more than likely work loose again).
When checking prices for welding a boat SHOP AROUND! Prices may differ greatly. I use a small local shop that does quaility work and they are VERY REASONABLE!
Good Luck..............