Do modern day Buddhists still believe in the magical siddhis? From Wikipedia on the Visuddhimagga and Siddhi:

Aṇimā: the ability to become smaller than the smallest, reducing one's
body to the size of an atom or even become invisible.
Mahimā: the ability to become infinitely large, expanding one's body
to an infinitely large size.
Laghimā: the ability to become weightless or lighter than air.
Prāpti: the ability to instantaneously travel or be anywhere at will.
Prākāmya: the ability to achieve or realize whatever one desires.
Īśiṭva: the ability to control nature, individuals, organisms, etc.
Supremacy over nature and ability to force influence upon anyone.
Vaśiṭva: the ability to control all material elements or natural
forces.
Kāma-avasayitva (per Kṣemarāja and Vyasa): satisfaction, suppression
of desire, or (as Yatrakāmāvasāyitva) wishes coming true.


Is it possible for a modern Buddhist to believe in what sounds like
fantasy or magical powers (Rddhi) ?
Do any current day Buddhists practice these?
Are the eight classical Siddhis or Rddhi now rejected or reinterpreted ?
Is it possible for a modern Buddhist to believe in what sounds like fantasy or magical powers (Rddhi) ?
If there is a belief in these things, then there exists conceptual artefacts. I certainly don't believe in them. In Buddhism, if they occur for you, - and they can occur in some quite powerful ways - they can be seen to have only an indicative value, both for personal practice and in how you skilfully integrate with others. Personally, I was plagued by these things, but given the correct attention, they served my practice well.
Do any current day Buddhists practice these?
Off the top of my head, Daniel Ingram is quite vocal about the siddhis. Whilst he is certainly an interesting character, I would find it difficult to recommend many of his practice methods to others for many reasons. For one, he teaches Buddhist practices that are far too powerful and disproportionately off-centre from the eightfold path for the average run-of-the-mill person, and some of those practices can't even be considered Buddhist in nature.
Are the eight classical Siddhis or Rddhi now rejected or reinterpreted ?
They are neither rejected nor reinterpreted. The approach to the siddhis from the suttas tells us two things: 1) to much wrong attention towards the siddhis is not helpful towards the primary task at hand: the ending of suffering. 2) the siddhis are only symptomatic of the primary task at hand. In other words, they are a sign that the radiant mind is opening further.

Then Venerable Anuruddha went up to Venerable Sāriputta, and exchanged
greetings with him. When the greetings and polite conversation were
over, he sat down to one side and said to him:
“Here’s the thing, Reverend Sāriputta. With clairvoyance that is
purified and surpasses the human, I survey the entire galaxy. My
energy is roused up and unflagging, my mindfulness is established and
lucid, my body is tranquil and undisturbed, and my mind is immersed in
samādhi. But my mind is not freed from the defilements by not
grasping.”
“Well, Reverend Anuruddha, when you say: ‘With clairvoyance that is
purified and surpasses the human, I survey the entire galaxy,’ that’s
your conceit. And when you say: ‘My energy is roused up and
unflagging, my mindfulness is established and lucid, my body is
tranquil and undisturbed, and my mind is immersed in samādhi,’ that’s
your restlessness. And when you say: ‘But my mind is not freed from
the defilements by not grasping,’ that’s your remorse. It would be
good to give up these three things. Instead of focusing on them, apply
your mind to the deathless.”
After some time Anuruddha gave up these three things. Instead of
focusing on them, he applied his mind to the deathless. Then
Anuruddha, living alone, withdrawn, diligent, keen, and resolute, soon
realized the supreme culmination of the spiritual path in this very
life. He lived having achieved with his own insight the goal for which
gentlemen rightly go forth from the lay life to homelessness.
He understood: “Rebirth is ended; the spiritual journey has been
completed; what had to be done has been done; there is no return to
any state of existence.” And Venerable Anuruddha became one of the
perfected.
With Anuruddha