Discounting Duru (away on loan and San Diego have an option to buy), Lucas Houghton is the best right back currently in the U21s. Wouldn't say he is particularly close, but sometimes, needs must. I think in the absence of Alebiosu they are more likely to either play TGH there, or move Miller to wing back and bring in Litherland/Atcheson at centre back.
Going forwards, Theo Pitt can play at either right or left back, and has looked a standout at U18 level (as well as featuring for our U21s & Wales U17s) this season despite being an U16, and should hopefully sign scholarship terms next summer. Arbitro is a particular fan I believe
Discounting Tyjon who is more a 10 anyway., and may well not be here come summer, Higgins is probably the current big hope at striker, think he turns 17 either this month or next, so will be able to sign a pro deal then, but has struggled with injury/fitness this season. Joseph is a possible shout and he does have a massive physical presence. Boggan always looks a goal threat for the U21s, but has struggled for game time during two separate loan spells in the 7th tier now
Mbonzale also looks good as striker, has featured for our U18s a few times this year, but is still at school, so that is not happening in the immediate future, and that's assuming he isn't poached before he is eligible to sign scholarship terms (not sure if that is either this upcoming summer or next, he was playing U12s football 3 or 4 years ago)