Sharp PC 4502 Battery Substitute?

Hello, folks -- My first computer, a portable Sharp PC 4502, needs a substitute battery, now that its original Yuasa battery is no longer manufactured. Other batteries exist almost identical in power specifications, but not in dimensions. The Yuasa battery had the following specifications: NP4.2-6H 6V, 4.2Ah -- dimensions: 1 7/8" x 2" x 4 5/8"; sealed lead-acid. The AC adaptor (which still works fine, luckily) has the following specifications: IN: 120V AC 60Hz 23 W; OUT: 10V DC 1.2A. (The computer has two 720k floppy drives, no hard drive, a back-lighted 640x200 LCD screen, V20 CPU clocked at 7.11 MHz, and the only peripheral I've used with it is my Okidata ML 182 Turbo 9-pin printer that I still share with three other computers on a data switch. I bought this computer in 1989 through DAMARK when it was already close to being obsolete; it was manufactured around 1987.) I still have the original dead battery, and it's stamped with 8710281 on its side; the second (nearly identical) Yuasa battery is stamped with 9410211, and I'm barely able to keep it charged for any meaningful length of time anymore. Does anyone know of a suitable substitute battery that would satisfy the above specifications? This has been a very interesting machine from an important era in pioneer portable computers that I think would be worthwhile preserving in working condition. Although I can run the machine with its AC adaptor, it would be a pain to keep that constantly plugged in to avoid the setup screen that pops up when turned on without a battery (and always have to reset everything). And it would be odd, too, to have an oversized battery connected to it from outside of its case, and inconvenient (to say the least). Thank you for any suggestions! Jerry... on his IBM PC/AT 5170 Model 339 | My laptop computer's a ***** 9600kbps/30MB HD/512k RAM/8 MHz | Tandy TRS-80 Model 100 Net-Tamer V 1.11.2X - Registered