The M777 is not already almost 20 years old. The modern American howitzer is the M109A7, which the Americans are quickly procuring, while upgrading their M109A6's to the newest standard. The M777 served its role, but the Americans are moving on from it as quickly as they can produce more M109A7's. A 20 year old weapon is not modern by American Standards.
As for Leopard tanks, there are hundreds in storage, around the world. Between 2A4, 2A5, and 2A6 variants, the West could cobble together 500 of them if they had the actual desire to defeat Russia. Germany has a couple hundred active and a couple hundred in storage. Spain has a couple hundred active, and more in storage. Norway, Poland, Portugal, Turkey, Greece, The Netherlands, Denmark, Austria, etc.... They all have them in storage and significant quantities could be cobbles together.
The Americans have more than 1,100 Patriots. If they wanted them to be sent to Ukraine, they could make it happen. They could take them from a variety places where they are currently operational. Some of the assets that they are currently defending could easily receiver other air defense systems instead. 50 is less than 5% of what they have.
1000 Humvees with TOW's, 50. CAL and grenade launchers would be a drop in the bucket for the USA. They have more than 100,000 Humvees and they're replacing almost all of them with their Oshkosh JLTVs. The USA has already announced around 1000 of them to Ukraine in packages and the Ukrainians are putting them to good use. 1000 more would retrofit several more battalions of high mobility infantry and keep the pressure on the Russians to keep up in the deployment of assets... You create teams of them with TOW missiles and Javelin armed soldiers in the Turret mount, hunting Russia tanks and quality IFV's. A cheap and effective trade off for the Ukrainians.
Of course the influx of Western weapons into Ukraine is hampered by the speed of training. You would, obviously, have to phase in this material over the course of months. You'd also have to set up additional training camps in Europe to selectively train Ukrainians how to use these weapons in mass quantities. It's highly doable, however. Europeans are already slated to train North of 25,000 Ukrainians outside of Ukraine in Q4 2022 and throughout 2023. Sending them back, gradually, the above weapons / systems wouldn't be particularly hard.
Let's say the NATO / EU spends a collective $100-$125 Billion U.S dollars between tomorrow and the end of 2023. I'm talking the USA, Canada, Australia, New Zealand, Britain, South Korea, the entire EU and the Scandinavian countries that are about to join NATO. Do you think that's a high price to pay to effectively neuter the Russian Army for a generation and deeply hurt the Russian economy along the way? That's the best deal they're ever going to get, without putting their military personnel in harm's way.
All it requires is the political desire to make it happen. If you follow the inventories and production capacity of the countries that form the alliance that is currently supplying Ukraine, you'd realize that the above list I posted is not far fetched, rather, a substantial logistical hurdle. The West has plenty enough equipment to make Russia's life miserable, despite all the bellyaching about Western military inventories.