The Yak-3 and the SAAB-21 were the only two fighters successfully* engine-swapped into jet powered platforms. There were varying degrees of success with these ideas, but for the most part, they were just interim measures until they learned how to design and build their own jet fighter from a clean sheet without raiding parts bins.
At first, the Yak-15 was constructed from the Yak-3, the Russian counter to the P-51.
Dropping the Klimov-M105 engine off the nose and mounting a captured Jumo-004 engine in a lower place provided a very attractive package in my opinion. The performance was reasonably assuring of the idea. It was a start and it was also the first Russian jet to fly in early 1946. It retained it’s tail dragging configuration with heat shielding and a solid-steel wheel required to combat the exhaust heat.
After the design proved capable and able to perform with a reasonable degree, they quickly spawned more developments: Yak-17, 19 and finally the -23, over three-hundred built but withdrawn in the late 1950’s. The configuration was short-lived.
Now, The SAAB-21:
Twin-boomed pusher design lent itself ideally for the experiment. They had to raise the tail a bit, but otherwise it was simply dropping in a new engine, mounts, and sheet-metal work. Trading 1,500 hp of Daimler German engine for 3,000 pounds of thrust from a DeHavilland goblin engine gave it enough push.
Flying by 1947 but not in service until 1950, SAAB already had another design that quickly outdated it. The J29, a purely new design, also entered service at the same time. The 21r worked very well at the time for ground attack and training to the new style of jet power.
*As the Yak-15, the SAAB-21r was also more of an interim design until they figured out how to properly build jet power. They’re success could be based on the designs not exhibiting deathtraps and explosions and in-flight break-ups but mostly, the ability of of these planes to teach their nations what a real jet fighter requires.