You ask the impossible, for two reasons:
(1) Simply citing an economist who says we can afford it wouldn't be proof. It would be "argument from authority" which is a classic logical fallacy.
(2) If I were to cite an economist who took the position that we can afford it, that would be prima facie proof, in your mind, that the economist was biased.
Towards the second point, I could offer up the names of Jonathan Gruber, Lawrence Summers, Robert Reich, Paul Krugman, Joseph Stiglitz, Thomas Piketty, Paul Collier, Kenneth Arrow, etc., as prominent economists who believe universal health insurance is affordable for the US. That list includes many of the world's leading economic minds, including multiple winners of the Nobel Prize for economics. But since they all believe in universal health insurance, I suspect none of those names will work for you. If I'm wrong, let me know, and I'll happy to share examples of their writing. For example, Paul Krugman is arguably the most respected living economist among other economists, and won the Nobel Prize in the subject. If you'd accept him as an expert in economics, I can cite many examples of him arguing in favor of universal health insurance.
If you recall, I used your own source -- the Mercatus Center (I assume that was your source, anyway, since you cited their number and haven't denied it). Even using that source's figure for the ten-year cost (which we can assume to err on the high side for the reasons I enumerated), it would STILL be cheaper than our current system. If we can afford the current system, we can definitely afford Medicare for all, since it would be cheaper.
If we insisted on keeping our radically low taxes, while still affording universal health insurance, then the cost is much higher. The increased hit on the federal government would be as much as $1.6 trillion per year, average (using the net of the Mercatus number and the current spending it would displace), and it's hard to find $1.6 trillion in savings in our budget. The biggest area of discretionary spending is the military budget, which is around $717 billion. If we reduced that to a more reasonable $250 billion, that wouldn't even cover a third of the savings we'd need to find. Most other spending areas are already extremely lean by wealthy-nation standards, so we can't find many savings there without seriously diminishing returns. Basically, we'd be cutting already-emaciated budgets, greatly harming the people. Assuming you're not OK with breaking past promises (e.g., defaulting on debts or refusing to pay up on our promises for Social Security), we're not going to find the rest of that money in further cuts without wrecking the society. So, boosting our radically low taxes is the way to go.
As for how much we'd have to raise them, I already addressed that. Raising them from 26% of GDP to 34% of GDP would do it. That could be done by increasing all taxes by about 31% (a proportional increase of 31 percent, not adding 31 percentage points). So, if your current effective tax rate is 10%, it would go to 13.1%. Etc.