Walmart U-1 lawn tractor battery, $35-40, and it has lots of cranks for those cold mornings.
Here's one on Ebay, less than 12 pounds, https://www.ebay.com/itm/Raion-Power-12V-18AH-SLA-Battery-for-Swisher-24-HP-Kawasaki-Riding-Mower/223157761727?epid=13031287835&hash=item33f53d36bf:g:~scAAOSwXI1cVF9M
Those added webs make the strength of the assembly several times more than the tubing alone. As gusset plates, they take all the bending and are immensely stronger that the original, so any weakening due to rust pits is completely gone. Looks like you added maybe 1.5 lbs to the truss?
Many of us do not use the tach gauge, the Tiny Tach is much more accurate and much more reliable. Beware of the china tiny tach copies, they can be very unreliable. You have a beautiful airplane, good luck!
Just to be sure data was what runs our decisions and not internet rumors, here is some real data, drawn from an Australian report of 6 years of light airplane experience (they were hunting Jibaru issues) that compares Continental, Lycoming, Rotax and Jibaru experience. The report is here : https://www.atsb.gov.au/media/5769864/ar-2013-107-final-report.pdf Note how similar Rotax is to Lycoming and Continental, hardly a "piece of rubbish waiting to kill you" frankly. Note that a Lycoming engine can be a little more of a safety issue than a Rotax, and that the total engine failure probability of a Rotax is only about 20% higher than a Continental or Lycoming.
What East TN Flyer doesn't know about engines has built an entire fleet of good, light, inexpensive and capable engines like the 582 and its cousins. Who picked the two strokes for the Avid line? The designer of the family, a near genius as an airplane designer and a guy who created the Avid/Kitfox light sport fleet, that's who. For those new guys who look at this site for advice and discussion, pls know that many of us completely disagree with East TN Flyer.
ChrisBolkan is right on, and for a shimmying tail wheel, the fix is to put more torque on the pivot bolt, which adds friction damping to the tail wheel rotation. Usually there is a wavy washer below the bolt head that can be sequentially crushed until shimmy disappears, see item 3 in the below print.
Rotax 582 Replacement starter (Rotax PN 995430) choices: Cheapest (same part number) $65!! , https://www.discountstarterandalternator.com/p/new-starter-ski-doo-snowmobile-410-207-500-995430/ What I bought, $195 https://www.ebay.com/itm/233052881422 Aircraft Spruce, $262 https://www.aircraftspruce.com/pages/ep/starters_xrotax/gplheavydutystrtrmtr.php Rotax Pricing $903 (but this is a full kit with housing and gear, not a fair comparison) https://www.air-techinc.com/product_detail.php?pid=2442
I had to replace my starter and decided to0 make a rear access hole instead of pulling the engine. Here is the story with pictures. http://avidfoxflyers.com/index.php?/topic/6443-how-to-pop-in-a-new-starter/#comment-58495
TJay, Yep, the 2 strokes have great power to weight, but poor fuel economy and the mess of the mixed oil. I think the internet myth about 2 stroke reliability is just a myth. I saw some data from Australia when the authorities were studying the Jab failures, where the 2 strokes had just about the same reliability as the typical Lycoming or Continental.
Let me add the 91 HP Rotax 670 to this list. I pulled the 582, its 2 bladed Culver wood prop and the 6 qt oil tank, added the 670 and its exhaust, a Warp Drive 3 bladed prop, a belly radiator, and a Highwing LLC landing gear (same wheels and tires), and a double tail wheel spring. The airplane weighs 21 lbs more after those mods. I would bet the 91 HP 670 is about 13 to 15 lbs heavier than the 65 HP 582, including the belly radiator and prop.