Well it looks like cvet doesn't care anymore about fixing older versions so we'll need to use other options.
Can't you migrate current version to newest or update step by step? I guess you have a lot of tasks to complete but web client or android client are promising idea. It all depends how many people just develop engine, probably not many I guess.
The engine is closed source and step from revision 404 or 412 to even the one around 500 is a MASSIVE amount of work and changes. Yet we're not even speaking about 600+ where everything is completely redone and changed from the very base.
Regarding Android/iOS/Web client the game is unplayable on devices with screen smaller than 10 inches and resolution 1280x720. I've already did prove it running it on first generation Nexus 7 where it had like 3 fps with SDL "emulation" and now on Windows 10 tablet.