I have one of those small after market starter button rivited to my frame and some times the starter wont work.A mate had a look at it for me and said its the starter solinoid thats on the way out.He said I could get away with wiring a beefier starter button and hooking the wires up to the solinoid some where.Because it will still work if you put a screwdriver across the terminals.It might be a bit of a hasle to get a new solinoid because the bike is made up from parts over a number of years and models.What do you reckon?

Strip the solenoid down by removing  the black cap, remove the copper terminals and file all the burn marks off them and do the same to the disc, be careful not to break the cap tightening the terminals its very brittle. clean all other terminals, you can get a heavy duty starter button from repco  and run a wire from your battery + to the button then from the button to the solenoid, you have to be careful as this way the bike can be started in gear as you have bypassed the neutral switch. The starter button sends power to the solenoid generating a magnetic field and drawing the disc up to the copper terminals completing the circuit from the battery to the starter motor.

If the solenoid is faulty, then bypassing the starter relay will clearly be of no use at all.
I really do not know why anyone would ever do that. All you need is a basic bosch relay to do the job with a starter button (switch) rated (almost) as low as you like.
If it actually is a solenoid problem, then it may indeed be fixable. No matter how many different year models you sourced parts for the bike from, buying a new generic solenoid will be very easy, and relatively inexpensive.
The fact that the starter drives when the 2 large terminals are crossed neither proves the solenoid good nor bad.
If you are crossing the big and small terminal, and the starter is driving correctly, then you have clearly proven the solenoid good.
If you actually mean the starter relay is faulty, then just replace it.