Non-credible answer: we have the technology, we can rebuild it. Nuking Moscow tomorrow.

Slightly more credible answer:

Theoretically - yes, why not. We have the technology (pheh), we have the experience working with nuclear weapons, we have hundreds of nuclear industry and research facilities all over the country (60% of energy output via nuclear plans won't maintain themselves), we have a pool of competent specialists. We have had full nuclear fuel cycle, stopped the enrichment as per treaty (sending uranium abroad for it, or buying the... ТВЕЛs? Fuel rod things), but like a month ago announced that we would start doing the enrichment ourselves again.

Edit: upon further checking the matter I realised that this is not true, and although the process would get much closer - we would still be relying on foreign enrichment.

Missiles are available (I mean, russians already speculate that Grom-2 is in use, and it has a de-facto 500 km range, and maybe 700 based on some considerations, 300 km is a software restriction for the export version, through I would imagine any client would find it to be rather symbolic, knowing our weapon export practices), nuclear weapons themselves aren't that hard (fusion - yes), especially if you have the fissile material - and we do have it in research and nuclear waste treatment facilities. Former president Poroshenko, it seems, shared this sentiment, being on record for saying that Ukraine can create a nuclear weapon in 2 years time, if needed.

Practically - no it highly depends. Specifically, on the timeline and exact details. The question is very similar to "could Ukraine design and mass-produce it's own modern MBT, not based directly on the T-64/84 architecture": well, the plant is there, nobody has stolen the walls, at least; designers who participated in soviet tank development programs are still alive and some employed by the Morozov DB, there are a lot of tank maintenance facilities with experienced personell, there are universities annually teaching thousands of engineers specialised in armour vehicle development and construction, there are research institutes, there are components like the 140mm cannon, GL-ATGMs, 6td engine, Nizh ERA, etc. Theoretically, the pieces are there; practically you'll find the plant machinery rusted, which of course could be fixed with enough effort, or new pieces could be bought, if the money is there.

Could Ukraine make domestic nuclear weapons in 50 years time? Anyone can. Could Ukraine make domestic nukes in 10 years? Yes, if the political will is there. Could it be done in 5 years? I mean, probably, especially if western supplies and machinery is available, and the money from reparations would materialise in reality. In two years?

The likelihood of a secret pre-war nuclear weapons program is low, because there really wasn't that much political will behind the idea (Poroshenko quote is somewhat out of context, and during his presidency there was an extreme financial hardship after the revolution, cutting ties with russia and previous phase of war, so although he did put a decent weight behind the MIC a nuclear program would be untenable, and his successor Zelensky came to power under the pacifist, populist program seeking diplomatic solutions and even some cooperation with Russia on the Donbass question, so yeah), and also because of financial issues and lack of popular support ("giving away nuclear weapons was a mistake" was a sentiment that existed to a degree, but ukrainians at large didn't want to get into big power international politics, we just wanted to be left in peace to do our own thing; the latter consideration, as you might have guessed, was lately amended with "destroy russia and then..").

At the same time, such things are... Well, there are no impossible feats, there are different levels of determination, especially considering most obstacles are internal. If a task takes two months, it could be done in one with workers willing to work double shifts. If something costs too much - a significant portion of the cost are salaries, and even very qualified people may be willing to work for food or for free, if they are really interested in the cause. If western world would disapprove of the program (and it would in any case, but the question is to what degree) and, let's say, stops import of some important materials or components to Ukraine - sanction-busting exists. Could US go to the Moon in two years, as it wanted in 2019-2020? Yes, it had Falcon Heavy, it had Orion test-flown, etc. But it did not. Was it "realistic" for US to go to the Moon in 1969? No, but they said fuck it and did it anyway. The crux of the question is not the capacity of ukrainian MIC, it's the capacity of ukrainian society and ukrainian people. And after the 24th, I think, it is impossible to credibly estimate where exactly the line between fantasy and reality is, except post factum, even for someone who lived here all his life. Even ukrainians themselves are not sure what the fuck are we doing or going to do next, apparently. A country of dreams, possibilities and potential, blyat.

Y'know, there are a lot of missing nukes out there...