You can use the stock speakers without any issues. The stock amp is another story....
The stock head unit wont have rca outs, and the amp inputs for signal and power most likely are Molex connectors. Even if the impedances dont match, if you had the schematic for the amp....you could still use it after figuring out what pin goes to where.
Personaly: I would install the Kenwood and use the stock speakers, then add an aftermarket amp as time/money allow. This is the easiest and less time consuming way.
You could allways just leave it stock too. What fun is that though?