You may find on this forum historical comments related to sources of frames for setting smaller cooktops into larger cut-outs. In my particular case, I have continued using Electrolux clones to fit my original cutout. When my Kenmore failed ca. 2015, I constructed the table below to help determine options among potential easy to fit candidates. You might want to create a modern version.

In my case, I could have gone with the Bosch using an in-set frame built by local sheet metal shops using, e.g., water jet or laser cut plate to fill the difference in desired cut-out size while carrying load to the internal cabinet structure. I'm less sure about strength if bonding fill strips to the soapstone.

(Click to enlarge.)

We took the specifications on our Wolf gas cooktop with us to the appliance store when selecting our Bosch induction cooktop for replacement. The cutout was, for practical purposes, the same.

Trim kits are available to fit a smaller unit into an existing larger cutout. Search online for sources.

I see the trim kits online. Not ideal but that would certainly solve the problem. I don't do anything DIY. Is this something a decent handyman could do? I'd want whoever is going to install it to be involved in measuring. I'm hoping current electric circuit is sufficient but will verify that and get it upgraded by an electrician if needed.

Your question is discussed in this houzz thread: https://www.houzz.com/discussions/6412916/induction-cooktops

What is the brand and model of the radiant cooktop you want to replace?

I replaced a radiant cooktop with induction. The existing circuit is 30 amps, less than the new cooktop's requirement. Decided the existing circuit is fine, since I'm not likely to have most of the burners on high or boost simultaneously.

@suburbanmd Under the NEC (NFPA 70), circuits should be provisioned such that their continuous duty load is not more than about 80% of their rated load. (Under usual code enforcement, appliance installations also have to conform to their installation guides. Code enforcement is thinking about what could be done during the lifetime of the cooktop, not what your cooking style will draw.) Eighty percent of 240 Vac x 30A is about 5.8 kVA. 36-inch induction cooktops typically operate near 10 kVA, and that is with only 3 of 5 hobs on boost. Mine is rated at 41A, and is on a 50A circuit.

While you may or may not have a fire hazard, depending on myriad factors related to cable heating and heat dissipation along with meeting specified terminal torques, fire hazard will primarily depend on the circuit breaker reliably disconnecting at the rated current of your wiring, without any help from the electrical load's rating. Please keep in mind that your insurance company and its lawyers won't be motivated to give you the benefit of the doubt if they have an excuse that you handed to them.

@wdccruise - I don't know the brand - not moving in until next month. I read the other thread and there was some helpful info in there though I'm still not so sure what kind of contractor I should get to do this project. I'm not going to be in an area with retailers that specialize in appliances and even that does not ensure you end up with an installation person who knows what they're doing especially if the cutout size needs to be tweaked.

Thanks @wdccruise - supposedly the seller has all the owners manuals for everything so that would help with the cutout dimensions. Problem is if the cooktop I want doesn't fit, it's unlikely I'll want one that does even if there is one that fits. Oh well, I have a lot of moving to do before I will be ready for this project.

"if the cooktop I want doesn't fit, it's unlikely I'll want one that does..."

Fortunately, among appliances, differences among induction cooktops aren't tremendous and sometimes cooktops from several manufacturers will fit in the same size cutout.

Just reporting back on this. I finally decided on the Bosch Benchmark 30 inch. I found a great guy on NextDoor and he had actually recut someone's countertop to fit a different cooktop so I thought he'd be a good person to help me with mine. Fortunately the installation instructions for the old cooktop were in the house and he compared those specs with the new Bosch and said it would fit. I was still a little skeptical but he installed it this week and it's perfect. I guess I got lucky!