64 is the best age to start a new adventure. I am the same age, did road racing for 25 years, had to stop due to neck injuries and the crouched jockey race style, Then 2 years later ex pro speedway rider and TT racer Peter Boast set up a British flattrack series and slide school. I jumped at it, Changed my life. The first school we used 125 honda engined speedway bikes, i can asure you that is plenty to learn on. 5 crashes in the day and i was awarded the best newcomer prise which was 3 laps on full on 500 weslake speedway bike. Wow! that was it, i was hooked. 12 years on and im still in there, Won my first national in 2018 riding a home built Moto Morini V twin. I have done a few slide schools, One was with Chris Carr, It is worth doing as a beginner, We have had jonny lewis over here too. My present bike is a Honda XL350/410 in a C&J frame that i have put together. I have to say that bike is the business, totally different feel to the Morini. I would not put to much emphasis on big power until you can use it. The Morini gave out 28 rear wheel hp, The Honda 30hp, both with nice flat torque curves, I have beaten many big twins on these 2 bikes, I guess our small 400yrd tracks help somewhat. My priority has always been Handling, it is impossible to ride fast on an ill handling bike, Geometry and suspension are key. I would urge you to spend as much as you can on rear shocks that have more rebound damping than you can use, then dial it in with the springs chosen for your weight, i have seen so many bikes bucking and weaving coming out of the turns because of poor setup. Similarly with front, Brit forks are not up to much, later 750 forks can be made to work better, But the one big benifit i found for corner entry was to get the Rake & Trail equation right. In 66 when Triumph changed the the rake to 28 degrees. a lot of the American racers complained that the bikes didn't handle like the previous years and wanted the 65 frame which was 25 degrees, Aim for that angle if you can, and spend some money on yokes/triple trees that give a trail of around 4 inches, believe me it will be the best thing a novice could in invest in, some ceriani/Betor yokes are about right, There is an online trail calculator here, www.rbracing-rsr.com
trust me its worth the effort, the pro,s use adjustable offset yokes to dial the front end in. if you have a couple of slabs of 1" alloy make your own. I am presently build a 750 A65 and a 750 long rod triumph in trackmaster frames that i have stashed away as there will be a 750 and over class for next season. You may say pah! i cannot be bothered with all this, but if you fancy riding the ovals it will be worth it, have a spare set of wheels and you could tarmac and dirt track race on the same bike? All good clean fun!!