This sounds like an interesting DIY project. I've found a description of one such repair online at
When you say you have a new battery, do you have "a" 3V lithium coin battery or specifically the <strong>CR2354 with tabs</strong>?
The replacement ones with tabs are available from any respectable electronics parts distributor, such as Jameco or Newark. Additionally, you can buy a holder for CR2354, such as this one:
Any of these repairs will require some soldering skills, so if you're comfortable with a soldering iron, it is not at all a difficult job. Thanks to Ed, who posted very detailed pictures of his ECJ-S35S repair (link above), you can see how to open the body of the cooker and how to find the battery. Ed has opted to replace the original one with a large holder for AAA batteries, which had to hang from the back due to lack of space inside. Personally, I would not do it - I would install a 3V coin battery holder instead but either way, you can use his great illustrations to aid in your repair.
If you're not that comfortable with a soldering iron, look for electronics repair service, not necessarily small appliance repair service. If you still have electronics repair services around where you live, these guys should be best equipped to deal with this simple repair. It absolutely does not have to be Sanyo or Panasonic, literally anyone with a soldering iron can do it. If they are not sure, send them Ed's link for a reference.