What if the execution time for an alt coin script exceeds the block time?
You shouldn't need to worry about this ever happening.

There are no loop opcodes in scripts. Every script opcode can only be executed once, and the slowest opcode is probably ECDSA signature verification (because it involves EC_point_multiply). Even so, it only takes about 50~100μs on a single core of a modern CPU to process.

Let's assume a 1MB block (max size) contains 100% signature verification opcodes, that's 1 million signature verification and takes at most 100 seconds CPU time of a single core, this is still much lower than the block time. In practice each scriptSig is accompanied by at least 100 bytes of data (pubkey + ECDSA signature) so it probably takes less than 1 second of CPU time.