* Significant Modifications Required: The engine bays of Geo Metros (and other Geo models) are designed around specific engine sizes. A 1.0L engine is significantly smaller than a 1.3L. You'll likely need to fabricate new motor mounts, possibly alter the exhaust and intake systems extensively, and potentially modify the wiring harness.
* Performance Loss: A 1.0L engine will produce considerably less power and torque than the 1.3L. Your car's performance will suffer dramatically. Acceleration will be sluggish, and hill climbing will become a challenge.
* Cost vs. Benefit: The cost of parts, labor (unless you're a skilled mechanic), and potential unforeseen problems during the swap will likely exceed the value of the resulting vehicle. You might be better off buying a Geo with a 1.0L engine to begin with.
* Parts Availability: Finding parts for older Geo engines, especially the 1.0L, can be challenging.
In short, while it's not impossible, swapping a 1.3L for a 1.0L Geo engine is a highly impractical project that is unlikely to yield positive results. It's far more sensible to either rebuild or replace the 1.3L engine or find a different vehicle.
