Mitsubishi Repair Problems? Ask a Mechanic for Answers ASAP
If the belt broke, the engine definitely is not going to start until a new timing belt is installed; until that is done, it won' start.
However the odds of it surviving a timing belt break are very slim; the four cam design is a heavy valve bender and it is not unusual for even a break at idle to wipe out a dozen valves;
If the belt broke at higher driving speed, it is pretty much a guarantee that half or more of your valves.
That being said, the motor isn't necessarily ruined, it just isn't going to be a quick or very cheap fix; the damage rarely goes beyond valve damage, and even in worst case scenarios you may end up with some damaged valve guides,... all repairable by a cylinder head shop or full service repair shop
The pistons and the connecting rods are quite strong in this engine, and I have never heard of bottom end damage on a 3000GT from a timing belt failure.
Unfortunately refurbing the cylinder heads is not a cheap endeavor, as you can expect around $1300 or so in labor alone,a little less if the heads are sublet to a machine shop usually. Then you need to add in the cost of valves (around $20 each) and a head gasket set, etc.... it isn't a good situation
The first thing you need to do is install a new belt and spin the engine by hand, see if it rotates (it may bind depending on severity of valve damage). If it is able to spin over compeltely twice, next step is a compression test to see where we are at for valve sealing.
If the compression test comes up good, you may have dodged a major bullet. If you have low compression on any cylinder, that indicates a valve sealing issue, meaning the valve is bent an unable to close all the way.