It's much more difficult than that. The biggest issue is finding where exactly to make the changes. Even then, we don't necessarily know what the phenotype, the eventual outcome, of making a change will be.
There are two versions of each gene (because there are two of each chromosome), what often happens is that even if one version of the gene is defective, the other version is enough to compensate.
Now, say you have two individuals, each one has one working and one defective version of a gene. If you cross those individuals 25% of the offspring will have two copies of the "good" version, 50% will have one good and one defective copy, and 25% will be unlucky and have two versions of the defective version. This is just regular Mendelian genetics so you might have heard all this before. However, it sets up why crossing two clones can be risky.
So, in the above example 75% of offspring will be fine. Sounds pretty good, right? But that's only when considering one gene. A mammoth likely has tens of thousands of genes. This particular one can be carrying any number of defective versions of each of those genes. Crossing it with an exact genetic copy would mean that the risk of having an offspring with a genetic defect is that much higher.
However, it might not be all that bad. Entire viable populations have likely descended from something like a pregnant female floating on a raft to a new island. If they can extract blood from even one other mammoth, that might be just enough to create healthy enough offspring. The main barrier is the gestation time and generation time. Two fertile rats stranded on an island with plenty of food can reproduce fast enough to quickly create a good population that might be able to overcome low genetic diversity through sheer numbers. Trying to raise enough mammoths will be much more expensive and much more time-consuming.
There are two versions of each gene (because there are two of each chromosome), what often happens is that even if one version of the gene is defective, the other version is enough to compensate.
Now, say you have two individuals, each one has one working and one defective version of a gene. If you cross those individuals 25% of the offspring will have two copies of the "good" version, 50% will have one good and one defective copy, and 25% will be unlucky and have two versions of the defective version. This is just regular Mendelian genetics so you might have heard all this before. However, it sets up why crossing two clones can be risky.
So, in the above example 75% of offspring will be fine. Sounds pretty good, right? But that's only when considering one gene. A mammoth likely has tens of thousands of genes. This particular one can be carrying any number of defective versions of each of those genes. Crossing it with an exact genetic copy would mean that the risk of having an offspring with a genetic defect is that much higher.
However, it might not be all that bad. Entire viable populations have likely descended from something like a pregnant female floating on a raft to a new island. If they can extract blood from even one other mammoth, that might be just enough to create healthy enough offspring. The main barrier is the gestation time and generation time. Two fertile rats stranded on an island with plenty of food can reproduce fast enough to quickly create a good population that might be able to overcome low genetic diversity through sheer numbers. Trying to raise enough mammoths will be much more expensive and much more time-consuming.