I am a Sukhoi 29 owner for 20 years now, I have never flown a Yak 54 but owned a yak Technoavia SP 55 which bears a nearly identical wing and the same engine.
The Yak 54 is an all metal construction cantilever wing; Its performance (Roll rate /Vertical penetration) is far from that of a Su-29. BUT it is a wonderfully easy and forgiving airplane to fly; Easy to master with appropriate training and with a tailwheel experience. It's tremendous value for $$$$$.
It handles like a fighter and its an excellent airshow performer.
It's only limitation is that it could not compete in unlimited aerobatic category but in good hands is a great performer. It's heavier and less agile than a Sukhoi.
It requires patience and time to keep it flying, like a vintage car it needs care and attention. The radial engine is wonderful and delivers tons of torque with an orgasmic sound.
Consider minimum consumption in reduced cruise 55/60 lit/hour. Aerobatics 80/100 litters.
The issue may be registration, you may be able to pass it experimental. If you are mechanically inclined and you can get a good deal, think about it seriously
You will not be disappointed....