Heat Exchange Tank has a serious leak. Can I bypass that system with a Large Water heater and us the circulation pump, demand, and safty features of existing unit???

Dont do it MAN.

Water tanks have no where near the recovery as a boiler.....I'm laughing because I cant believe you would even fathom this project.
Please let an HVAC contractor help you. It may be cheaper than you think to solve your problem.

Hollywood.