I'll start with the bad news - if it's the CD100 model without electric start then it's a no-go unless you replace the engine (I'm inferring this from your statement "customize my old bike with a self start"). You won't be able to retro fit a starter to it as the cases and internals will not be configured for one. However, if the engine does have electric start, read on. :)
At it's simplest, your circuit should be made to activate a low current relay that can itself be tapped into the feed to the starter relay (in parallel with the starter button wiring) so it can energise it and turn the starter motor.
You then need to consider safety aspects - has the bike got any safety interlocks in the starter wiring? Look for clutch switches (clutch must be pulled in to start the bike) and side stand switches (side stand must be up to put it in gear). These are there to prevent starting the bike in dangerous circumstances (in gear or about to ride off with the side stand down). Don't try and work around these, work with them and fit them if they are not already on the bike. You don't want to try and impress your friends by remote starting the bike when it is accidentally in gear and it goes tumbling off the stand onto the floor!
Good luck with the project!