Paul the biggest piece of advice I can give you is to stay away from IC engines unless you have ALOT of experience with them and even think long and hard about using them.

To compress a spring you are going to want a lot of torque. At low revs an IC engine just does not have a huge amount of torque. It gets this around the 2/3 rpm range which means you need a clutch. You will also need to be able to start it from inside the arena without touching it incase the engine stalls in battle (and believe me it will do). This means an electric starter which is adding more complexity to the design.

Electric motor, far few working parts to go wrong, it has all its torque at lows revs (where you want it) and they are generally cheaper and easier to maintain.

The biggest headache I had when on robot wars was the petrol engine we used in typhoon 2 to spin the disc. Dont me get wrong, there are nice advantages such as being able to have a seperate energy source for the weapon and your fuel not counting in the weight limit but then you also have to consider if the events you will enter will be able to run an IC engine. A lot of indoor events cant.

Ok I have waffled a bit but hopefully you can pick out some nuggets from all the waffle.