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I'm sorry but the tape is no longer listed on ebay. The instructable was written 5 years ago and the seller is not listed any more. I can't help you any further.
The heating tapes will definitely NOT withstand 650 degrees Celsius. I believe their upper limit is about 450 Celsius.
The coffee percolator would not have sufficient heating capacity for that volume of liquid, nor would the stirrer be powerful enough. I would suggest you have a look at these instructables which may satisfy your requirements.https://www.instructables.com/id/Programmable-Temperature-Controller-Hot-Plate/https://www.instructables.com/id/DIY-StirrerHot-Plate/
The original coffee percolator was probably designed to heat about 1 litre to approximately 70 deg, C. To heat larger volumes to higher temperatures would require bypassing the thermostat and connecting the element via a suitable SCR energy regulator ( 1000 Watts ) and temperature controller. Have a look at my Instructable for a temperature controlled laboratory heater for a typical setup. I would advise a maximum temperature of 100 deg.C otherwise some of the plastic components and the fan will overheat. The device is intended mainly for use with organic solvents with boiling points below 80 deg. C. Highly flammable solvents like diethyl ether should not be heated in open containers, but in a water bath with a reflux conderser.
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