Here's a pretty decent summary of whether the EU can replace the US military not just in the war in Ukraine but in general (eg. the Americans completely desert Europe and move all their troops out - this is a very unlikely scenario and would take a decade to do at which point the next King of America would/could reverse it).
It adjusts for purchasing power such that Russia is spending about the same as the EU and points out that the Russian war machine is up and running so they're cranking up production of ammunition, tanks, drones etc.
It doesn't tackle the economic part though - that Russia's economy is a house of cards compared to the EU. In a short war Russia can win but it doesn't seem plausible that Russia can win over the long term - the hope would be that if the Americans do withdraw from Ukraine that it happens slow enough that the EU can get up to speed.